Coelenterazine analogs and manufacturing method thereof

ABSTRACT

There has been a need for coelenterazine analogs that exhibit luminescence properties different from those of known coelenterazine analogs. The present invention provides the compound represented by general formula (1).

TECHNICAL FIELD

The present invention relates to coelenterazine analogs, a method for manufacturing the same, and so on.

BACKGROUND ART

The phenomenon of bioluminescence was observed in some living species, and was based on the chemical reaction of a luciferin (a luminescence substrate) and a luciferase (an enzyme catalyzes the luminescence reaction) in vivo. A number of researches including the researches for identification of luciferin and luciferase and for elucidation of the luminesence mechanism in a molecular level have been performed inside and outside of Japan.

In recent years, bioluminescence is used as a tool for biological research. In addition, the applied researches in the medical field including high through-put screening (HTS) of drugs, intramolecular imaging, etc., have been intensively developed on the basis of the principle of bioluminescence.

Fireflies, sea pansies Renilla, sea fireflies Cypridina, deep-sea shrimps Oplophorus, luminescent microorganisms, etc. are known as representative bioluminescent organisms that produce bioluminescence. The jellyfish Aequorea victoria is also a bioluminescent animal, but the bioluminescence of the jellyfish is not produced by the luciferase reaction. The luminescence is produced by the Ca²⁺-triggered reaction of the photoprotein of aequorin, the complex of substrate-enzyme-molecular oxygen. It is known that many organisms utilize the compound having an imidazopyrazinone skeleton as a luminescence substrate in the bioluminescence system.

Among them, coelenterazine (CTZ) is a compound commonly used as a luminescence substrate (luciferin) for aequorin which is a photoprotein from jellyfish, or for luciferases from some bioluminescent organisms such as sea pansies Renilla, etc. Therefore, many findings of CTZ have been accumulated.

In fact, approximately 50 types of coelenterazine analog (CTZ analog) have been synthesized heretofore, and the substrate specificity for some of them has been examined in several bioluminescence systems (cf., e.g., Non-Patent Literatures 1 to 5).

CITATION LIST Non-Patent Literature

-   [Non-Patent Literature 1] Shimomura O. et al., Biochem. J. 251,     405-410 (1988) -   [Non-Patent Literature 2] Shimomura O. et al., Biochem. J. 261,     913-920 (1989) -   [Non-Patent Literature 3] Inouye S. & Shimomura O., Biochem.     Biophys. Res. Commun. 233, 349-353 (1997) -   [Non-Patent Literature 4] Inouye S. & Sasaki S., Protein Express.     Purif. 56, 261-268 (2007) -   [Non-Patent Literature 5] Inouye S. & Sahara Y., Biochem. Biophys.     Res. Commun. 265, 96-101 (2008)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Under the foregoing situations, coelenterazine analogs having different luminescence properties from those of known coelenterazine analogs have been sought.

Means of Solving the Problems

The present inventors have conducted extensive investigations to solve the foregoing problems. As a result, the inventors have found that coelenterazine analogs having methyl, trifluoromethyl, methoxy or ethyl in place of hydroxy on the benzene ring at the position 2 of coelenterazine possess luminescence properties, which are different from those of known coelenterazine analogs, and have come to attain the present invention.

That is, the present invention provides coelenterazine analogs, methods for producing coelenterazine analogs, and so on, which are shown below.

(1) A compound represented by general formula (1) below:

wherein:

R¹ is hydrogen, hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl;

R² is hydrogen, hydroxy, a halogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl;

R³ is hydrogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxyl;

R⁴ is a substituted or unsubstituted aryl, a substituted or unsubstituted arylalkyl, a substituted or unsubstituted arylalkenyl, an alkyl which may optionally be substituted with an alicyclic group, an alkenyl which may optionally be substituted with an alicyclic group, an alicyclic group, or a heterocyclic group;

R⁵ is hydrogen or a substituted or unsubstituted alkyl;

X¹ is hydrogen, hydroxy, a halogen, an alkoxyl or amino; and,

X² is hydrogen or hydroxy;

with the proviso that when R² and R³ are hydrogen, R¹ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl,

when R¹ and R³ are hydrogen, R² is hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl, or,

when R¹ and R² are hydrogen, R³ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxyl.

(2) The compound according to (1) above, wherein R¹ is hydrogen, hydroxy, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, trifluoromethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, in the general formula (1).

(3) The compound according to (1) or (2) above, wherein R² is hydrogen, hydroxy, fluorine, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, trifluoromethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, in the general formula (1).

(4) The compound according to any one of (1) to (3) above, wherein R³ is hydrogen, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, in the general formula (1).

(5) The compound according to any one of (1) to (4) above, wherein R⁴ is phenyl, p-hydroxyphenyl, benzyl, α-hydroxybenzyl, phenylethyl, phenylvinyl, cyclohexyl, cyclohexylmethyl, cyclohexylethyl, methyl, ethyl, propyl, 2-methylpropyl, 2-methylpropenyl, adamantylmethyl, cyclopentylmethyl or thiophen-2-yl, in the general formula (1).

(6) The compound according to any one of (1) to (5) above, wherein, in the general formula (1):

R¹ is hydrogen, methyl, ethyl, trifluoromethyl or methoxy; R² is hydrogen, hydroxy, methyl or methoxy; R³ is hydrogen or methyl; R⁴ is benzyl; R⁵ is hydrogen; X¹ is hydroxy; and, X² is hydrogen;

with the proviso that when R² and R³ are hydrogen, R¹ is methyl, ethyl, trifluoromethyl or methoxy,

when R¹ and R³ are hydrogen, R² is hydroxy, methyl or methoxy, or,

when R¹ and R² are hydrogen, R³ is methyl.

(7) The compound according to (6) above, which is represented by formula below.

(8) The compound according to (6) above, which is represented by formula below.

(9) The compound according to (6) above, which is represented by formula below.

(10) The compound according to (6) above, which is represented by formula below.

(11) The compound according to (6) above, which is represented by formula below.

(12) The compound according to (6) above, which is represented by formula below.

(13) The compound according to (6) above, which is represented by formula below.

(14) The compound according to (6) above, which is represented by formula below.

(15) A process for producing a compound represented by general formula (1) below:

wherein R¹, R², R³, R⁴, R⁵, X¹ and X² are the same as defined below, which comprises reacting a compound represented by general formula (2) below:

wherein:

R⁴ is a substituted or unsubstituted aryl, a substituted or unsubstituted arylalkyl, a substituted or unsubstituted arylalkenyl, an alkyl which may optionally be substituted with an alicyclic group, an alkenyl which may optionally be substituted with an alicyclic group, an alicyclic group, or a heterocyclic group:

R⁵ is hydrogen or a substituted or unsubstituted alkyl;

X¹ is hydrogen, hydroxy, a halogen, an alkoxyl or amino; and,

X² is hydrogen or hydroxy;

with a compound represented by general formula (3) below:

wherein:

R¹ is hydrogen, hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl;

R² is hydrogen, hydroxy, a halogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl; and,

R³ is hydrogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxy;

with the proviso that when R² and R³ are hydrogen, R¹ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl,

when R¹ and R³ are hydrogen, R² is hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl, and,

when R¹ and R² are hydrogen, R³ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxyl.

(16) The process according to (15) above, wherein R¹ is hydrogen, hydroxy, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, trifluoromethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, in the general formula (1).

(17) The process according to (15) or (16) above, wherein R² is hydrogen, hydroxy, fluorine, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, trifluoromethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, in the general formula (1).

(18) The process according to any one of (15) to (17) above, wherein R³ is hydrogen, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, in the general formula (1).

(19) The process according to any one of (15) to (18) above, wherein R⁴ is phenyl, p-hydroxyphenyl, benzyl, t-hydroxybenzyl, phenylethyl, phenylvinyl, cyclohexyl, cyclohexylmethyl, cyclohexylethyl, methyl, ethyl, propyl, 2-methylpropyl, 2-methylpropenyl, adamantylmethyl, cyclopentylmethyl or thiophen-2-yl, in the general formula (1).

(20) The process according to any one of (15) to (19) above, wherein, in the general formula (1):

R¹ is hydrogen, methyl, ethyl, trifluoromethyl or methoxy, R² is hydrogen, hydroxy, methyl or methoxy, R³ is hydrogen or methyl, R⁴ is benzyl, R⁵ is hydrogen, X¹ is hydroxy, and, X² is hydrogen,

with the proviso that when R² and R³ are hydrogen, R¹ is methyl, ethyl, trifluoromethyl or methoxy,

when R¹ and R³ are hydrogen, R² is hydroxy, methyl or methoxy, and,

when R¹ and R² are hydrogen, R³ is methyl.

(21) A method for producing a calcium-binding photoprotein, which comprises contacting the compound according to any one of (1) to (14) above with an apoprotein of the calcium-binding photoprotein to obtain the calcium-binding photoprotein.

(22) A method for detecting or quantifying a calcium ion, which comprises using the calcium-binding photoprotein produced by the method according to (21) above.

(23) A method for analyzing a physiological function or enzyme activity, which comprises performing a bioluminescence resonance energy transfer (BRET) assay using as a donor protein the calcium-binding photoprotein produced by the method according to (21) above.

(24) A method for measuring a transcription activity or detecting an analyte, which comprises using the compound according to any one of (1) to (14) above, and a luciferase derived from Renilla sp., Oplophorus sp. or Gaussia sp.

(25) A method for analyzing a physiological function or enzyme activity, which comprises performing a bioluminescence resonance energy transfer (BRET) assay using the compound according to any one of (1) to (14) above and a luciferase derived from Renilla sp., Oplophorus sp. or Gaussia sp. as a donor protein.

(26) The method according to (24) or (25) above, wherein Renilla sp. is Renilla reniformis.

(27) The method according to (26) above, wherein the luciferase derived from Renilla reniformis comprises a polypeptide consisting of the amino acid sequence of SEQ ID NO: 14.

(28) The method according to (24) or (25) above, wherein Oplophorus sp. is Oplophorus grachlorostris.

(29) The method according to (28) above, wherein the luciferase derived from Oplophorus grachlorostris comprises a polypeptide consisting of the amino acid sequence of SEQ ID NO: 16.

(30) The method according to (24) or (25) above, wherein Gaussia sp. is Gaussia princeps.

(31) The method according to (30) above, wherein the luciferase derived from Gaussia princeps comprises a polypeptide consisting of the amino acid sequence of SEQ ID NO: 18.

(32) A kit for measuring a transcription activity or detecting an analyte, comprising the compound according to any one of (1) to (14) above and a luciferase derived from at least one organism selected from the group consisting of Renilla sp., Oplophorus sp. and Gaussia sp.

(33) A kit for analyzing a physiological function or enzyme activity, comprising the compound according to any one of (1) to (14) above, a luciferase derived from at least one organism selected from the group consisting of Renilla sp., Oplophorus sp. and Gaussia sp. and at least one selected from the group consisting of an organic compound and a fluorescent protein, utilizing the principle of intermolecular interaction by a bioluminescence resonance energy transfer (BRET) assay.

Effect of the Invention

The present invention provides novel coelenterazine analogs. Coelenterazine analogs in a preferred embodiment of the present invention exhibit the luminescence properties which are different from those of known coelenterazine analogs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the relationship between the regeneration time and luminescence intensity of semi-synthetic aequorins.

FIG. 2 shows the luminescence patterns for semi-synthetic aequorins.

FIG. 3 shows the emission spectra of semi-synthetic aequorins by addition of calcium ions.

FIG. 4 shows the relationship between the initial luminescence intensity of semi-synthetic aequorins and calcium ion levels.

FIG. 5 shows the emission spectra of Oplophorus luciferase by addition of coelenterazine or its analogs.

FIG. 6 shows the emission spectra of Gaussia luciferase by addition of coelenterazine or its analogs.

FIG. 7 shows the emission spectra of Renilla luciferase by addition of coelenterazine or its analogs.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention is described in detail.

1. Coelenterazine Analog of the Invention

The present invention provides the following compound (coelenterazine analog of the present invention) represented by general formula (1) below.

wherein:

R¹ is hydrogen, hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl;

R² is hydrogen, hydroxy, a halogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl;

R³ is hydrogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxyl;

R⁴ is a substituted or unsubstituted aryl, a substituted or unsubstituted arylalkyl, a substituted or unsubstituted arylalkenyl, an alkyl which may optionally be substituted with an alicyclic group, an alkenyl which may optionally be substituted with an alicyclic group, an alicyclic group, or a heterocyclic group;

R⁵ is hydrogen or a substituted or unsubstituted alkyl;

X¹ is hydrogen, hydroxy, a halogen, an alkoxyl or amino; and,

X² is hydrogen or hydroxy;

with the proviso that when R² and R³ are hydrogen, R¹ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl;

when R¹ and R³ are hydrogen, R² is hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl; and,

when R¹ and R² are hydrogen, R³ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxyl.

The “alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group” in R¹ is, for example, an unsubstituted straight or branched alkyl having 1 to 4 carbon atoms, or a straight or branched alkyl having 1 to 4 carbon atoms which is substituted with, e.g., 1 to 10 alicyclic groups. Examples of the alicyclic group include cyclohexyl, cyclopentyl, adamantyl, cyclobutyl, cyclopropyl, etc. Preferably, the alicyclic group is cyclohexyl, cyclopentyl, adamantyl, etc. The “alkyl which may optionally be substituted with an alicyclic group” is, for example, methyl, ethyl, propyl, 2-methylpropyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, cyclobutylmethyl, cyclopropylmethyl, etc., preferably, methyl, ethyl, propyl, 2-methylpropyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, and the like. In some embodiments of the present invention, the “alkyl which may optionally be substituted with an alicyclic group” is a straight alkyl which may optionally be substituted with an alicyclic group and examples are methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, etc.

The “alkoxyl” in R¹ is, for example, a straight or branched alkoxy having 1 to 6 carbon atoms. Examples of the “alkoxy” are methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentyloxy, iso-pentyloxy, sec-pentyloxy, 1,1-dimethylpropyloxy, 1,2-dimethylpropoxy, 2,2-dimethylpropyloxy, n-hexoxy, 1-ethylpropoxy, 2-ethylpropoxy, 1-methylbutoxy, 2-methylbutoxy, iso-hexoxy, 1-methyl-2-ethylpropoxy, 1-ethyl-2-methylpropoxy, 1,1,2-trimethylpropoxy, 1,1,2-trimethylpropoxy, 1-propylpropoxy, 1,1-dimethylbutoxy, 1,2-dimethylbutoxy, 2,2-dimethylbutoxy, 2,3-dimethylbutyloxy, 1,3-dimethylbutyloxy, 2-ethylbutoxy, 1,3-dimethylbutoxy, 2-methylpentoxy, 3-methylpentoxy, hexyloxy, etc. In some embodiments of the invention, the “alkoxy” is methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, preferably, methoxy.

In a preferred embodiment of the invention, R¹ is hydrogen, hydroxy, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, trifluoromethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy. In a more preferred embodiment of the invention, R¹ is hydrogen, methyl, ethyl, trifluoromethyl or methoxy.

The “halogen” in R² is, for example, fluorine, chlorine, bromine or iodine. In a preferred embodiment of the present invention, the “halogen” is fluorine.

The “alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group” in R² is, for example, an unsubstituted straight or branched alkyl having 1 to 4 carbon atoms, or a straight or branched alkyl having 1 to 4 carbon atoms which is substituted with, e.g., 1 to 10 alicyclic groups. Examples of the alicyclic group include cyclohexyl, cyclopentyl, adamantyl, cyclobutyl, cyclopropyl, etc. Preferably, the alicyclic group is cyclohexyl, cyclopentyl, adamantyl, etc. The “alkyl which may optionally be substituted with an alicyclic group” is, for example, methyl, ethyl, propyl, 2-methylpropyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, cyclobutylmethyl, cyclopropylmethyl, etc., preferably, methyl, ethyl, propyl, 2-methylpropyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, etc. In some embodiments of the present invention, the “alkyl which may optionally be substituted with an alicyclic group” is a straight alkyl which may optionally be substituted with an alicyclic group and examples are methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, etc.

The “alkoxyl” in R² is, for example, a straight or branched alkoxy having 1 to 6 carbon atoms, and examples are methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentyloxy, iso-pentyloxy, sec-pentyloxy, 1,1-dimethylpropyloxy, 1,2-dimethylpropoxy, 2,2-dimethylpropyloxy, n-hexoxy, 1-ethylpropoxy, 2-ethylpropoxy, 1-methylbutoxy, 2-methylbutoxy, iso-hexoxy, 1-methyl-2-ethylpropoxy, 1-ethyl-2-methylpropoxy, 1,1,2-trimethylpropoxy, 1,1,2-trimethylpropoxy, 1-propylpropoxy, 1,1-dimethylbutoxy, 1,2-dimethylbutoxy, 2,2-dimethylbutoxy, 2,3-dimethylbutyloxy, 1,3-dimethylbutyloxy, 2-ethylbutoxy, 1,3-dimethylbutoxy, 2-methylpentoxy, 3-methylpentoxy, hexyloxy, etc. In some embodiments of the invention, the “alkoxy” is methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, preferably, methoxy.

In a preferred embodiment of the invention, R² is hydrogen, hydroxy, fluorine, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, trifluoromethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy. In a more preferred embodiment of the invention, R² is hydrogen, hydroxy, methyl or methoxy.

The “alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group” in R³ is, for example, an unsubstituted straight or branched alkyl having 1 to 4 carbon atoms, or a straight or branched alkyl having 1 to 4 carbon atoms which is substituted with, e.g., 1 to 10 alicyclic groups. Examples of the alicyclic group include cyclohexyl, cyclopentyl, adamantyl, cyclobutyl, cyclopropyl, etc. Preferably, the alicyclic group is cyclohexyl, cyclopentyl, adamantyl, etc. The “alkyl which may optionally be substituted with an alicyclic group” is, for example, methyl, ethyl, propyl, 2-methylpropyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, cyclobutylmethyl, cyclopropylmethyl, etc., preferably, methyl, ethyl, propyl, 2-methylpropyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, etc. In some embodiments of the invention, the “alkyl which may optionally be substituted with an alicyclic group” is a straight alkyl which may optionally be substituted with an alicyclic group, and examples include methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, etc.

The “alkoxyl” in R³ is, for example, a straight or branched alkoxy having 1 to 6 carbon atoms. Examples of the “alkoxy” are methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentyloxy, iso-pentyloxy, sec-pentyloxy, 1,1-dimethylpropyloxy, 1,2-dimethylpropoxy, 2,2-dimethylpropyloxy, n-hexoxy, 1-ethylpropoxy, 2-ethylpropoxy, 1-methylbutoxy, 2-methylbutoxy, iso-hexoxy, 1-methyl-2-ethylpropoxy, 1-ethyl-2-methylpropoxy, 1,1,2-trimethylpropoxy, 1,1,2-trimethylpropoxy, 1-propylpropoxy, 1,1-dimethylbutoxy, 1,2-dimethylbutoxy, 2,2-dimethylbutoxy, 2,3-dimethylbutyloxy, 1,3-dimethylbutyloxy, 2-ethylbutoxy, 1,3-dimethylbutoxy, 2-methylpentoxy, 3-methylpentoxy, hexyloxy, etc. In some embodiments of the invention, the “alkoxy” is methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, preferably, methoxy.

In a preferred embodiment of the invention, R³ is hydrogen, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy. In a more preferred embodiment of the invention, R³ is hydrogen or methyl.

The “substituted or unsubstituted aryl” in R⁴ is, for example, an aryl having 1 to 5 substituents or an unsubstituted aryl. The substituent is, for example, at least one selected from the group consisting of a halogen (fluorine, chlorine, bromine or iodine, etc.), hydroxy, an alkyl having 1 to 6 carbon atoms, an alkoxyl having 1 to 6 carbon atoms, amino and a dialkylamino having 1 to 6 carbon atoms. In some embodiments of the invention, the substituent is hydroxy. Specific examples of the “substituted or unsubstituted aryl” are phenyl, p-hydroxyphenyl, p-aminophenyl, p-dimethylaminophenyl, etc., preferably, phenyl, p-hydroxyphenyl, etc. In some embodiments of the invention, the “substituted or unsubstituted aryl” is an unsubstituted aryl, e.g., phenyl, etc.

The “substituted or unsubstituted arylalkyl” in R⁴ is, for example, an arylalkyl having 7 to 10 carbon atoms, which is substituted with 1 to 5 substituents, or an unsubstituted arylalkyl having 7 to 10 carbon atoms. The substituent includes, for example, a halogen (fluorine, chlorine, bromine or iodine, etc.), hydroxy, an alkyl having 1 to 6 carbon atoms, an alkoxyl having 1 to 6 carbon atoms, amino, a dialkylamino having 1 to 6 carbon atoms, etc. Examples of the “substituted or unsubstituted arylalkyl” are benzyl, α-hydroxybenzyl, phenylethyl, p-hydroxybenzyl, p-dimethylaminobenzyl, etc., preferably, benzyl, α-hydroxybenzyl, phenylethyl, etc. In some embodiments of the invention, the “substituted or unsubstituted arylalkyl” is benzyl.

The “substituted or unsubstituted arylalkenyl” in R⁴ is, for example, an arylalkenyl of 8 to 10 carbon atoms having 1 to 5 substituents, or an unsubstituted arylalkenyl having 8 to 10 carbon atoms. The substituent includes, for example, a halogen (fluorine, chlorine, bromine or iodine, etc.), hydroxy, an alkyl having 1 to 6 carbon atoms, an alkoxyl having 1 to 6 carbon atoms, amino, a dialkylamino having 1 to 6 carbon atoms, etc. Examples of the substituted or unsubstituted arylalkenyl” are phenylvinyl, p-hydroxyphenylvinyl, p-dimethylaminophenylvinyl, etc. In some embodiments of the present invention, the “substituted or unsubstituted arylalkenyl” is an unsubstituted arylalkenyl, e.g., phenylvinyl, etc.

The “alkyl which may optionally be substituted with an alicyclic group” in R⁴ is, for example, an unsubstituted straight or branched alkyl having 1 to 4 carbon atoms, or a straight or branched alkyl having 1 to 4 carbon atoms which is substituted with, e.g., 1 to 10 alicyclic groups. Examples of the alicyclic group include cyclohexyl, cyclopentyl, adamantyl, cyclobutyl, cyclopropyl, etc. Preferably, the alicyclic group is cyclohexyl, cyclopentyl, adamantyl, etc. Examples of the “alkyl which may optionally be substituted with an alicyclic group” are methyl, ethyl, propyl, 2-methylpropyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, cyclobutylmethyl, cyclopropylmethyl, etc., preferably, methyl, ethyl, propyl, 2-methylpropyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, etc. In some embodiments of the present invention, the “alkyl which may optionally be substituted with an alicyclic group” is a straight alkyl which may optionally be substituted with an alicyclic group, and examples include methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, etc.

The “alkenyl which may optionally be substituted with an alicyclic group” in R⁴ is, for example, an unsubstituted straight or branched alkenyl having 2 to 6 carbon atoms, or a straight or branched alkenyl having 2 to 6 carbon atoms which is substituted with, e.g., 1 to 10 alicyclic groups. Examples of the alicyclic group include cyclohexyl, cyclopentyl, adamantyl, cyclobutyl, cyclopropyl, etc. Preferably, the alicyclic group is cyclohexyl, cyclopentyl, adamantyl, etc. Examples of the “alkenyl which may optionally be substituted with an alicyclic group” include vinyl, 1-propenyl, 2-propenyl, 1-butenyl, 2-butenyl, 3-butenyl, 2-methylpropenyl, etc., preferably, 2-methylpropenyl, etc.

The “alicyclic group” in R⁴ includes, for example, cyclohexyl, cyclopentyl, adamantyl, cyclobutyl, cyclopropyl, etc. Preferably, the alicyclic group is cyclohexyl, etc.

The “heterocyclic group” in R⁴ includes, for example, a group derived from a 5- to 7-membered ring containing, in addition to carbon atoms, 1 to 3 atoms selected from the group consisting of N, O and S as the atoms constituting the ring and bonded via carbon atoms, a group formed by fusing 2 or more of such rings and bonded via carbon, or a group formed by fusing such a ring to a benzene ring and bonding via carbon atoms. Examples of the “heterocyclic group” are thiophen-2-yl, 2-furanyl, 4-pyridyl, etc. In some embodiments of the present invention, the “heterocyclic group” is a heterocyclic group containing sulfur, e.g., thiophen-2-yl.

In a preferred embodiment of the present invention, R⁴ is phenyl, p-hydroxyphenyl, benzyl, α-hydroxybenzyl, phenylethyl, phenylvinyl, cyclohexyl, cyclohexylmethyl, cyclohexylethyl, methyl, ethyl, propyl, 2-methylpropyl, 2-methylpropenyl, adamantylmethyl, cyclopentylmethyl or thiophen-2-yl. In a more preferred embodiment of the present invention, R⁴ is benzyl.

The “substituted or unsubstituted alkyl” in R⁵ is, for example, an alkyl of 1 to 6 carbon atoms having, e.g., 1 to 6 substituents, or an unsubstituted alkyl having 1 to 6 carbon atoms. The substituent is, for example, at least one selected from the group consisting of a halogen (fluorine, chlorine, bromine or iodine, etc.), hydroxy, carboxyl, an alkyl having 1 to 6 carbon atoms, an alkoxyl having 1 to 6 carbon atoms, amino and a dialkylamino having 1 to 6 carbon atoms. In some embodiments of the invention, the substituent is hydroxy. Specific examples of the “substituted or unsubstituted alkyl” are methyl, 2-hydroxyethyl, carboxymethyl, 3-hydroxypropyl, etc., preferably, methyl, 2-hydroxyethyl, etc.

In a preferred embodiment of the present invention, R⁵ is hydrogen, methyl or 2-hydroxyethyl. In a more preferred embodiment of the present invention, R⁵ is hydrogen.

The “halogen” in X¹ is, for example, fluorine, chlorine, bromine or iodine. In a preferred embodiment of the present invention, the “halogen” is fluorine.

The “alkoxyl” in X¹ is an alkoxyl having, e.g., 1 to 6 carbon atoms, and examples include methoxy, ethoxy, propoxy, isopropoxy, cyclopropoxy, tert-butyloxy, etc. In a preferred embodiment of the present invention, the “alkoxyl” is methoxy.

In a preferred embodiment of the present invention, X¹ is hydrogen, hydroxy, fluorine, methoxy or amino. In a more preferred embodiment of the present invention, X¹ is hydroxy.

In a preferred embodiment of the present invention, X² is hydrogen.

In some embodiments of the present invention, the following symbols represent as follows in the general formula (1):

R¹ is hydrogen, methyl, ethyl, trifluoromethyl or methoxy; R² is hydrogen, hydroxy, methyl or methoxy; R³ is hydrogen or methyl; R⁴ is benzyl; R⁵ is hydrogen; X¹ is hydroxy; and, X² is hydrogen;

with the proviso that when R² and R³ are hydrogen, R¹ is methyl, ethyl, trifluoromethyl or methoxy;

when R¹ and R³ are hydrogen, R² is hydroxy, methyl or methoxy; and,

when R¹ and R² are hydrogen, R³ is methyl.

In an embodiment of the present invention, the compound represented by general formula (1) is a compound represented by the formula below.

In another embodiment of the present invention, the compound represented by general formula (1) is a compound represented by the formula below.

In a still another embodiment of the present invention, the compound represented by general formula (1) is a compound represented by the formula below.

In a still another embodiment of the present invention, the compound represented by general formula (1) is a compound represented by the formula below.

In a still another embodiment of the present invention, the compound represented by general formula (1) is a compound represented by the formula below.

In a still another embodiment of the present invention, the compound represented by general formula (1) is a compound represented by the formula below.

In a still another embodiment of the present invention, the compound represented by general formula (1) is a compound represented by the formula below.

In a still another embodiment of the present invention, the compound represented by general formula (1) is a compound represented by the formula below.

Coelenterazine analogs in an embodiment of the present invention exhibit the luminescence properties, which are different from those of known coelenterazine analogs (e.g., h-coelenterazine, n-coelenterazine, i-coelenterazine, etc.). Coelenterazine analogs in some embodiments of the present invention become relatively good luminescence substrates for at least one luciferase selected from the group consisting of Oplophorus luciferase, Renilla luciferase and Gaussia luciferase. Coelenterazine analogs in a preferred embodiment of the present invention become relatively good luminescence substrates for Oplophorus luciferase, Renilla luciferase and Gaussia luciferase.

2. Process for Producing Coelenterazine Analog of the Invention

The compound represented by general formula (1) (coelenterazine analog of the invention) can be produced as follows.

That is, the compound represented by general formula (1) can be produced by reacting the compound represented by general formula (2) below:

(wherein R⁴, R⁵, X¹ and X² are as defined above) with the compound represented by general formula (3) below:

(wherein R¹, R² and R³ are as defined above), whereby the compound represented by general formula (1) can be obtained.

The compound represented by general formula (2) can be prepared by known processes. For example, the compound represented by general formula (2) can be prepared, e.g., by the process described in Kishi, Y. et al., Tetrahedron Let., 13, 2747-2748 (1972), or Adamczyk, M. et al., Org. Prep. Proced Int., 33, 477-485 (2001), or their modifications. More specifically, the compound represented by general formula (2) can be prepared as follows. That is, first, cyclization of a substituted phenylglyoxal aldoxime and a glycinonitrile derivative is carried out using a Lewis acid catalyst to form the pyrazine oxide. Subsequently, the pyrazine oxide is subjected to catalytic hydrogenation using Raney Ni, etc. as a catalyst to prepare the compound. Alternatively, the compound represented by general formula (2) can be prepared by conducting the Suzuki-Miyaura coupling reaction between a 2-amino-5-bromopyrazine derivative and a substituted phenylboronic acid pinacol ester.

The compound represented by general formula (3) can be prepared by known processes. For example, the compound represented by general formula (3) can be prepared, e.g., by the processes described in Adamczyk, M. et al., Synth. Commun., 32, 3199-3205 (2002), or Baganz, H. & May, H.-J. Chem. Ber., 99, 3766-3770 (1966) and Baganz, H. & May, H.-J. Angew. Chem., Int. Ed. Eng., 5, 420 (1966), or their modifications. More specifically, the compound represented by general formula (3) can be prepared as follows. That is, the compound represented by general formula (3) can be prepared either by reacting a substituted benzyl Grignard reagent with ethyl diethoxyacetate at a low temperature (−78° C.), or by reacting an α-diazo-α′-substituted phenyl ketone with tert-butyl hypochlorite in ethanol.

Herein, the solvent used for the process for producing the compound of the present invention represented by general formula (1) is not particularly limited and various solvents can be used. Examples of the solvent include dioxane, tetrahydrofuran, ether, methanol, ethanol, water, etc. These solvents can be used alone or as an admixture thereof.

In the process of producing the compound of the present invention represented by general formula (1), the reaction temperature and reaction time are not particularly limited and are, for example, 0° C. to 200° C. for 1 to 96 hours, room temperature to 150° C. for 3 to 72 hours, or 60° C. to 120° C. for 6 to 24 hours.

3. Method for Producing Calcium-Binding Photoprotein

The calcium-binding photoprotein of the invention can be produced or regenerated by contacting the compound represented by general formula (1) (coelenterazine analog of the invention) with the apoprotein of the calcium-binding photoprotein thereby to obtain the calcium-binding photoprotein.

As used herein, the term “contact” means that coelenterazine analog of the invention and the apoprotein of the calcium-binding photoprotein are allowed to be present in the same reaction system, and includes, for example, the apoprotein of the calcium-binding photoprotein being added to a container charged with coelenterazine analog of the invention, coelenterazine analog of the invention being added to a container charged with the apoprotein of the calcium-binding photoprotein, coelenterazine analog of the invention being mixed with the apoprotein of the calcium-binding photoprotein, and the like. In one embodiment of the present invention, the contact is carried out at a low temperature in the presence of a reducing agent (e.g., mercaptoethanol, dithiothreitol, etc.) and oxygen. More specifically, the photoprotein of the present invention can be produced or regenerated by the methods described in, e.g., Shimomura, O. et al. Biochem. J. 251, 405-410 (1988), Shimomura, O. et al. Biochem. J. 261, 913-920 (1989), and the like. The calcium-binding photoprotein of the present invention is present in such a state that a complex is formed between the peroxide of coelenterazine analog generated from coelenterazine analog of the invention and molecular oxygen and the apoprotein. Calcium ions are bound to the complex above to generate instantaneous luminescence and form coelenteramide analog, which is the oxide of coelenterazine analog, and carbon dioxide. The complex above is sometimes referred to as “the photoprotein of the present invention.”

The apoprotein used to produce the photoprotein of the present invention includes, for example, apoaequorin, apoclytin-I, apoclytin-II, apobelin, apomitrocomin, apomineopsin, apobervoin, and the like. In some embodiments of the present invention, the apoprotein is apoaequorin, apobelin, apoclytin-I, apoclytin-II, mitrocomin, etc., e.g., apoaequorin. These apoproteins may be obtained from natural sources or genetically engineered. Furthermore, the amino acid sequence may be mutated from the natural sequence by gene recombination technology, so long as the apoproteins are capable of producing the calcium-binding photoprotein.

The nucleotide sequences and amino acid sequences of the apoproteins of photoproteins obtained from the nature (natural apoproteins) are as follows. That is, the nucleotide sequence and amino acid sequence of natural apoaequorin are represented by SEQ ID NO: 1 and SEQ ID NO: 2. The nucleotide sequence and amino acid sequence of natural apoclytin-1 are represented by SEQ ID NO: 3 and SEQ ID NO: 4. The nucleotide sequence and amino acid sequence of natural apoclytin-II are represented by SEQ ID NO: 5 and SEQ ID NO: 6. The nucleotide sequence and amino acid sequence of natural apomitrocomin are represented by SEQ ID NO: 7 and SEQ ID NO: 8. The nucleotide sequence and amino acid sequence of natural apobelin are represented by SEQ ID NO: 9 and SEQ ID NO: 10. The nucleotide sequence and amino acid sequence of natural apobervoin are represented by SEQ ID NO: 11 and SEQ ID NO: 12.

The apoprotein mutated by recombinant technology is a protein selected from the group consisting of (a) to (c) below:

(a) a protein comprising the amino acid sequence of natural apoprotein in which 1 or more amino acids are deleted, substituted, inserted and/or added, and having the apoprotein activity or function of the calcium-binding photoprotein;

(b) a protein comprising an amino acid sequence which is 90% or more homologous to the amino acid sequence of natural apoprotein, and having the apoprotein activity or function of the calcium-binding photoprotein; and,

(c) a protein comprising an amino acid sequence encoded by a polynucleotide that hybridizes under stringent conditions to a polynucleotide consisting of a nucleotide sequence complementary to the nucleotide sequence of natural apoprotein, and having the apoprotein activity or function of the calcium-binding photoprotein.

Examples of the “natural apoprotein” described above are apoaequorin, apoclytin-I, apoclytin-II, apobelin, apomitrocomin, apomineopsin, apobervoin, etc. In an embodiment of the present invention, the apoprotein is apoaequorin, apoclytin-I, apoclytin-II, apobelin, apomitrocomin, etc., preferably apoaequorin. The amino acid sequences and nucleotide sequences of these natural apoproteins are as described above.

The “apoprotein activity or function of the calcium-binding photoprotein” means the activity or function that, e.g., the apoprotein binds to the peroxide of coelenterazine or the peroxide of a coelenterazine analog to produce the calcium-binding photoprotein. Specifically, “the protein binds to the peroxide of coelenterazine or the peroxide of a coelenterazine analog to produce the calcium-binding photoprotein” not only means that (1) the protein binds to the peroxide of coelenterazine or the peroxide of a coelenterazine analog to produce the photoprotein, but also means that (2) the protein is brought into contact with coelenterazine or its derivative in the presence of oxygen to produce a photoprotein (complex) comprising the protein and the peroxide of coelenterazine or the peroxide of a coelenterazine analog. As used herein, the term “contact” means that the protein and coelenterazine or its analog are allowed to be present in the same reaction system, and includes, for example, the protein being added to a container charged with coelenterazine or its analog, coelenterazine or its analog being added to a container charged with the protein, the protein being mixed with coelenterazine or its analog, and the like. The “coelenterazine analog” refers to a compound which is capable of constituting a calcium-binding photoprotein such as aequorin, etc., together with the apoprotein in the same manner as in coelenterazine. Examples of coelenterazine or its analog include, in addition to coelenterazine analogs of the present invention, coelenterazine, h-coelenterazine, f-coelenterazine, cl-coelenterazine, n-coelenterazine, cp-coelenterazine, ch-coelenterazine, hch-coelenterazine, fch-coelenterazine, e-coelenterazin, ef-coelenterazine, ech-coelenterazine, hcp-coelenterazine, and the like. Coelenterazine analogs of the present invention can be produced, e.g., by the processes described above or their modifications. The other coelenterazines or their analogs can be produced by the processes described in, e.g., Shimomura et al. (1988) Biochem. J. 251, 405-410, Shimomura et al. (1989) Biochem. J., 261, 913-920, Shimomura et al. (1990) Biochem. J., 270, 309-312, or their modifications. Alternatively, various types of coelenterazine analogs are commercially available from Chisso Corporation, Wako Pure Chemical Industry Co., Ltd. and Promega Inc., and these commercial products may also be used.

The range of “1 or more” in “the amino acid sequence in which 1 or more amino acids are deleted, substituted, inserted and/or added” described above is, for example, 1 to 20, 1 to 15, 1 to 10, 1 to 9, 1 to 8, 1 to 7, 1 to 6 (1 to several), 1 to 5, 1 to 4, 1 to 3, 1 to 2, and 1. In general, the less the number of amino acids deleted, substituted, inserted or added, the more preferable. In the deletion, substitution, insertion and addition of the amino acid residues described above, two or more may occur concurrently. Such regions can be acquired using site-directed mutagenesis described in “Sambrook J. et al., Molecular Cloning: A Laboratory Manual, Third Edition, Cold Spring Harbor Laboratory Press (2001),” “Ausbel F. M. et al., Current Protocols in Molecular Biology, Supplement 1-38, John Wiley and Sons (1987-1997),” “Nuc. Acids. Res., 10, 6487 (1982),” “Proc. Natl. Acad. Sci. USA, 79, 6409 (1982),” “Gene, 34, 315 (1985),” “Nuc. Acids. Res., 13, 4431 (1985),” “Proc. Natl. Acad. Sci. USA, 82, 488 (1985),” etc.

The range of “90% or more” in the “amino acid sequence which is 90% or more homologous” described above is, for example, 90% or more, 91% or more, 92% or more, 93% or more, 94% or more, 95% or more, 96% or more, 97% or more, 98% or more, 99% or more, 99.1% or more, 99.2% or more, 99.3% or more, 99.4% or more, 99.5% or more, 99.6% or more, 99.7% or more, 99.8% or more, or 99.9% or more. It is generally preferred for the numerical value indicating the degree of homology to be higher. The homology between amino acid sequences or nucleotide sequences can be determined using a sequencing program such as BLAST (see, e.g., Altzchul, S. F. et al., J. Mol. Biol., 215, 403 (1990), etc.) or the like. When BLAST is used, the default parameters for the respective programs are employed.

The “polynucleotide that hybridizes under stringent conditions” described above refers to a polynucleotide (e.g., DNA) which is obtained by, for example, colony hybridization, plaque hybridization or Southern hybridization using as the probe a polynucleotide consisting of a nucleotide sequence complementary to the nucleotide sequence of natural apoprotein or all or part of the polynucleotide encoding the amino acid sequence of natural apoprotein. Specific examples include a polynucleotide which can be identified by performing hybridization at 65° C. in the presence of 0.7 to 1.0 mol/L NaCl using a filter on which the polynucleotide from the colony or plaque is immobilized, then washing the filter at 65° C. with 0.1- to 2-fold SSC (saline-sodium citrate) solution (a 1-fold SSC solution is composed of 150 mmol/L sodium chloride and 15 mmol/L sodium citrate).

Hybridization may be performed in accordance with modifications of the methods described in manuals, e.g., Sambrook, J. et al.: Molecular Cloning: A Laboratory Manual, Third Edition, Cold Spring Harbor Laboratory Press (2001), Ausbel F. M. et al., Current Protocols in Molecular Biology, Supplement 1-38, John Wiley and Sons (1987-1997), Glover D. M. and Hames B. D., DNA Cloning 1: Core Techniques, A practical Approach, Second Edition, Oxford University Press (1995), etc.

As used herein, “stringent conditions” may refer to less stringent conditions, moderately stringent conditions and highly stringent conditions. The “less stringent conditions” are, for example, the conditions under 5×SSC, 5×Denhardt's solution, 0.5% (w/v) SDS and 50% (v/v) formamide at 32° C. The “moderately stringent conditions” are, for example, the conditions under 5×SSC, 5×Denhardt's solution, 0.5% (w/v) SDS and 50% (v/v) formamide at 42° C. The “highly stringent conditions” are, for example, the conditions under 5×SSC, 5×Denhardt's solution, 0.5% (w/v) SDS and 50% (v/v) formamide at 50° C. The more stringent the conditions are, the higher the complementarity required for double strand formation. Specifically, for example, under these conditions, a polynucleotide (e.g., DNA) of higher homology is expected to be obtained efficiently at higher temperatures, although multiple factors are involved in hybridization stringency, including temperature, probe concentration, probe length, ionic strength, time and salt concentration; those skilled in the art may appropriately choose these factors to realize a similar stringency.

Where a kit commercially available is used for the hybridization, for example, Alkphos Direct Labeling Reagents (manufactured by Amersham Pharmacia) may be used. In this case, incubation with a labeled probe is performed overnight in accordance with the protocol attached to the kit, the membrane is then washed with a primary wash buffer containing 0.1% (w/v) SDS at 55° C., and finally the hybridized DNA can be detected.

Other hybridizable polynucleotides include, as calculated by a sequencing program such as BLAST or the like using the default parameters, DNAs having a homology of approximately 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 88% or more, 90% or more, 92% or more, 95% or more, 97% or more, 98% or more, 99% or more, 99.3% or more, 99.5% or more, 99.7% or more, 99.8% or more, or 99.9% or more, to the polynucleotide encoding the amino acid sequence of the apoprotein. The homology of amino acid sequences or nucleotide sequences can be determined using the method described above.

The recombinant apoprotein which can be used in the present invention includes, for example, recombinant aequorin described in Shimomura, O. and Inouye, S. Protein Express. Purif. (1999) 16: 91-95, recombinant clytin-I described in Inouye, S. and Sahara, Y. Protein Express. Purif. (2007) 53: 384-389, recombinant clytin-II described in Inouye, S. J. Biochem. (2008) 143: 711-717, and the like.

The calcium-binding photoprotein thus obtained may be further purified. Purification of the calcium-binding photoprotein may be performed in a conventional manner of separation/purification. The separation/purification includes, for example, precipitation with ammonium sulfate, gel filtration chromatography, ion exchange chromatography, affinity chromatography, reversed phase high performance liquid chromatography, dialysis, ultrafiltration, etc., alone or in an appropriate combination thereof.

The photoprotein in an embodiment of the present invention exhibits the luminescence properties which are different from those of known photoproteins. The photoprotein in some embodiments of the present invention show less Ca²⁺ sensitivity when compared to the photoprotein containing natural coelenterazine, and are well suited for applications to a high-precision assay system in which Ca²⁺ level change in the system is used as an indicator, in the same manner as in the photoprotein containing i-CTZ or n-CTZ.

4. Application of Coelenterazine Analog of the Invention or the Photoprotein of the Invention (1) Use as Luminescence Substrate

Coelenterazine analog in some embodiments of the present invention emits light by the action of a luminescent catalyst and can thus be used as a luminescence substrate. Accordingly, the present invention provides a light-emitting method, which comprises contacting coelenterazine analog of the present invention with a luminescent catalyst. As used herein, the term “contact” means that coelenterazine analog of the invention and the luminescent catalyst are allowed to be present in the same reaction system, and includes, for example, the luminescent catalyst being added to a container charged with coelenterazine analog, coelenterazine analog being added to a container charged with the luminescent catalyst, coelenterazine analog being mixed with the luminescent catalyst, and the like.

The luminescent catalyst used for the light-emitting method of the present invention includes, for example, a luciferase derived from Oplophorus sp. (e.g., Oplophorus grachlorostris) (Oplophorus luciferase), a luciferase derived from Gaussia sp. (e.g., Gaussia princeps) (Gaussia luciferase), a luciferase derived from Renilla sp. (e.g., Renilla reniformis or Renilla muelleri) (Renilla luciferase), a luciferase derived from Pleuromamma sp. (Pleuromamma luciferase), or a luciferase derived from Metridia longa (Metridia luciferase). Coelenterazine analog in some embodiments of the present invention acts as a luminescence substrate for Oplophorus luciferase, Gaussia luciferase or Renilla luciferase. Coelenterazine analog in an embodiment of the present invention acts as a luminescence substrate for Oplophorus luciferase. Coelenterazine analog in another embodiment of the present invention acts as a luminescence substrate for Oplophorus luciferase, Gaussia luciferase and Renilla luciferase. These luminescent catalysts can be produced by the method described in, e.g., Shimomura et al. (1988) Biochem. J. 251, 405-410, Shimomura et al. (1989) Biochem. J. 261, 913-920, or Shimomura et al. (1990) Biochem. J. 270, 309-312, or modifications thereof. Alternatively, various products are commercially available from Chisso Corporation, Wako Pure Chemical Industry Co., Ltd. and Promega Inc., and these commercial products may also be used for the light-emitting method of the present invention.

Now, of Renilla luciferase, the nucleotide sequence and amino acid sequence of the luciferase derived from Renilla reniformis are represented by SEQ ID NO: 13 and SEQ ID NO: 14. Of Oplophorus luciferase, the nucleotide sequence and amino acid sequence of the luciferase derived from Oplophorus grachlorostris are represented by represented by SEQ ID NO: 15 and SEQ ID NO: 16. Furthermore, of Gaussia luciferase, the nucleotide sequence and amino acid sequence of the luciferase derived from Gaussia princeps are represented by SEQ ID NO: 17 and SEQ ID NO: 18.

In an embodiment of the present invention, Renilla luciferase is the luciferase derived from Renilla reniformis and comprises a polypeptide consisting of the amino acid sequence of, e.g., SEQ ID NO: 14. In another embodiment of the present invention, Oplophorus luciferase is the luciferase derived from Oplophorus grachlorostris and comprises a polypeptide consisting of the amino acid sequence of, e.g., SEQ ID NO: 16. In still another embodiment of the present invention, Gaussia luciferase is the luciferase derived from Gaussia princeps and comprises a polypeptide consisting of the amino acid sequence of, e.g., SEQ ID NO: 18.

When these luminescent catalysts are brought into contact with coelenterazine analog in some embodiments of the present invention, light is produced. The emission time is generally 0.01 to 1 hour. However, the emission time can be more prolonged or the emission time can be further shortened, depending upon conditions chosen.

(2) Detection or Quantification of Calcium Ions

The photoprotein of the present invention obtained as above is a photoprotein (holoprotein) that non-covalent bond is formed between the apoprotein and the peroxide of coelenterazine analog produced from coelenterazine analog and molecular oxygen and emits light by the action of calcium ions. Thus, the photoprotein of the invention can be used for the detection or quantification of calcium ions.

The detection or quantification of calcium ions may be performed by adding a sample solution directly to a solution of the photoprotein and measuring the luminescence generated. Alternatively, calcium ions may also be detected or quantified by adding a solution of the photoprotein to a sample solution and measuring the luminescence generated. The photoprotein described above may also be previously produced, before addition to the assay system for the detection or quantification of calcium ions, by contacting an aqueous solution of the apoprotein with coelenterazine analog of the present invention, which is provided for use. Alternatively, the photoprotein composed of the apoprotein and the peroxide of coelenterazine analog may be formed by contacting the apoprotein with coelenterazine analog in the assay system. The photoprotein formed is a complex (photoprotein) of the apoprotein and the peroxide of coelenterazine analog of the invention. The complex (i.e., the photoprotein of the present invention) described above emits light dependently on the calcium ion level.

The detection or quantification of calcium ions can be performed by measuring on a luminometer the luminescence of the photoprotein of the invention induced by calcium ions. Luminometers which can be used include commercially available instruments such as Centro LB 960 (manufactured by Berthold), etc. The calcium ion level can be quantified by preparing a luminescence standard curve for known calcium ion levels using the photoprotein.

Coelenterazine analog of the present invention may also be used to detect changes in intracellular calcium ion levels under physiological conditions, by preparing the photoprotein composed of the apoprotein and the peroxide of coelenterazine analog and directly introducing the photoprotein into the cell by means of microinjection, etc.

In addition to the introduction into a cell by means of microinjection, etc., coelenterazine analog of the present invention may also be used to form the photoprotein by intracellularly expressing an apoprotein gene (a polynucleotide encoding the apoprotein) to form the apoprotein within a cell and then adding coelenterazine analog of the present invention to the apoprotein thus formed from outside the cell.

Using the photoprotein of the invention thus introduced into or formed within the cell, changes in intracellular calcium ion levels in response to external stimuli (e.g., stimuli with a drug which is associated with a receptor) can also be measured.

(3) Use as Reporter Protein, etc. by Luminescence

The photoprotein of the present invention may also be used as a reporter protein to determine the transcription activity of a promoter, etc. A polynucleotide encoding the apoprotein is fused to a target promoter or other expression control sequence (e.g., an enhancer, etc.) to construct a vector. The vector is transformed to a host cell. Coelenterazine or its analog is brought into contact with the transformant. By detecting the luminescence from the photoprotein of the present invention, the activity of the target promoter or other expression control sequence can be determined. As used herein, the term “contact” means that a host cell and coelenterazine or its analog are allowed to be present in the same culture system or reaction system, and includes, for example, coelenterazine or its analog being added to a culture container charged with a host cell, a host cell being mixed with coelenterazine or its analog, a host cell being cultured in the presence of coelenterazine or its analog, and the like. Coelenterazine or its analog includes those described above, in addition to coelenterazine analog of the present invention.

Coelenterazine analog of the present invention may be used to determine the transcription activity of a promoter, etc. For example, a polynucleotide encoding a luminescent catalyst is fused to a target promoter or other expression control sequence (e.g., an enhancer, etc.) to construct a vector. The vector is transformed to a host cell. Coelenterazine analog of the present invention is brought into contact with the transformant. By detecting the luminescence from coelenterazine analog of the present invention, the activity of the target promoter or other expression control sequence can be determined. As used herein, the term “contact” means that a host cell and coelenterazine analog of the present invention are allowed to be present in the same culture system or reaction system, and includes, for example, coelenterazine analog being added to a culture container charged with a host cell, a host cell being mixed with coelenterazine analog, a host cell being cultured in the presence of coelenterazine analog, and the like. The luminescent catalyst includes those described above and is at least one selected from the group consisting of Renilla luciferase, Oplophorus luciferase and Gaussia luciferase.

The present invention further provides a kit used for measuring the transcription activity of a promoter, etc. The kit in some embodiments of the present invention comprises coelenterazine analog of the present invention and the luminescent catalyst. The kit in another embodiment of the present invention comprises the photoprotein of the present invention and coelenterazine or its analog. Reagents including coelenterazine or its analog, the luminescent catalyst, etc. may be dissolved in a suitable solvent and prepared into a form suitable for storage. At least one selected from the group consisting of water, ethanol, various buffer solutions and the like may be used as the solvent. The kit may additionally contain, if necessary, at least one selected from the group consisting of exclusive containers, other necessary accessories, instruction manuals, etc.

(4) Use as Detection Marker, etc. by Luminescence

The photoprotein of the present invention can be used as a detection marker by luminescence. The detection marker of the present invention can be used to detect an analyte in, e.g., immunoassay, hybridization assay, etc. The photoprotein of the present invention can be used in the form bound to an analyte (protein, nucleic acid, etc.) by methods conventionally used, such as chemical modification. Detection using such a detection marker can be carried out in a conventional manner. The detection marker of the invention may also be used, for example, by expressing as a fusion protein of the apoprotein and an analyte, inserting the fusion protein into a cell by means of microinjection, etc. and contacting the protein with coelenterazine analog of the invention to produce the photoprotein of the present invention, and then contacting the photoprotein with coelenterazine or its analog, thereby to determine distribution of the analyte described above. As used herein, the term “contact” means that a cell and coelenterazine analog, etc. of the present invention are allowed to be present in the same culture system or reaction system, and includes, for example, coelenterazine analog, etc. of the present invention being added to a culture container charged with a cell, a cell being mixed with coelenterazine analog, etc. of the present invention, a host cell being cultured in the presence of coelenterazine analog, etc. of the present invention, and the like. Examples of the “coelenterazine or its analog” are those described above, in addition to coelenterazine analog of the present invention.

The present invention provides a method for detecting an analyte in immunoassay, hybridization assay, etc., which comprises using coelenterazine analog and the luminescent catalyst. In this case, the luminescent catalyst can be used in the form bound to an analyte (protein, nucleic acid, etc.) by methods conventionally used, such as chemical modification. Detection using such a detection marker can be carried out in a conventional manner. The detection marker may also be used, for example, by expressing as a fusion protein of the luminescent catalyst and an analyte, inserting the fusion protein into a cell by means of microinjection, etc. and contacting the protein with coelenterazine analog of the invention, thus to determine distribution of the analyte described above. As used herein, the term “contact” means that a cell and coelenterazine analog of the present invention are allowed to be present in the same culture system or reaction system, and includes, for example, coelenterazine analog of the present invention being added to a culture container charged with a cell, a cell being mixed with coelenterazine analog of the present invention, a host cell being cultured in the presence of coelenterazine analog of the present invention, and the like. The luminescent catalyst includes those described above and is at least one selected from the group consisting of Renilla luciferase, Oplophorus luciferase and Gaussia luciferase.

Measurement of the distribution of such an analyte may be carried out using a detection method such as luminescence imaging, etc. Aside from the introduction into a cell by means of microinjection, etc., it is also possible to use the apoprotein by expressing the same within a cell.

The invention also provides a kit used for detecting an analyte in, e.g., immunoassay, hybridization assay, etc. The kit in some embodiments of the present invention comprises the photoprotein of the invention and coclenterazine or its analog. The kit in another embodiment of the present invention comprises coelenterazine analog of the invention and the luminescent catalyst. Reagents including coelenterazine or its analog, the luminescent catalyst, etc. may be dissolved in a suitable solvent and prepared into a form suitable for storage. At least one selected from the group consisting of water, ethanol, various buffer solutions and the like may be used as the solvent. The kit may additionally contain, if necessary, at least one selected from the group consisting of exclusive containers, other necessary accessories, instruction manuals, etc.

(5) Material for Amusement Supplies

The complex (photoprotein of the present invention) composed of the apoprotein and the peroxide of coelenterazine analog of the present invention emits light merely by binding to a trace amount of calcium ions. Therefore, the photoprotein of the invention can be advantageously used as a luminescence substrate in materials for amusement supplies. Examples of amusement supplies include luminescent bubble soap, luminescent ice, luminescent candies, luminescent paints, etc. The amusement supplies of the invention can be prepared in a conventional manner.

(6) Bioluminescence Resonance Energy Transfer (BRET) Method

Coelenterazine analog in some embodiments of the present invention emits light by the action of the luminescent catalyst as described above and can thus be used for the method of analyses, including an analysis of biological functions, an analysis (or measurement) of enzyme activities, etc., based on the principle of intermolecular interactions by the bioluminescence resonance energy transfer (BRET) method. In addition, the photoprotein of the present invention can also be used for the method of analyses such as an analysis of biological functions, measurement of enzyme activities, etc., based on the principle of intermolecular interactions by the bioluminescence resonance energy transfer (BRET) method.

For example, using coelenterazine analog of the present invention in some embodiments of the invention and the luminescent catalyst as donor proteins and an organic compound or a fluorescent protein as an acceptor, the interactions between the proteins can be detected by causing bioluminescence resonance energy transfer (BRET) between them. Herein, the luminescent catalyst includes those described above and is at least one selected from the group consisting of Renilla luciferase, Oplophorus luciferase and Gaussia luciferase. Alternatively, using the photoprotein of the invention as a donor protein and an organic compound or fluorescent protein an acceptor, the interactions between the proteins can be detected by causing bioluminescence resonance energy transfer (BRET) between them. In an embodiment of the present invention, the organic compound used as an acceptor is Hoechst 3342, Indo-1, DAPI, etc. In another embodiment of the present invention, the fluorescent protein used as an acceptor is a green fluorescent protein (GFP), a blue fluorescent protein (BFP), a mutant GFP fluorescent protein, phycobilin, etc. In a preferred embodiment of the present invention, the physiological functions to be analyzed include an orphan receptor (in particular, a G-protein coupled receptor), apoptosis, transcription regulation by gene expression, etc. In a preferred embodiment of the present invention, the enzyme to be analyzed is protease, esterase, kinase, or the like.

Analysis of the physiological functions by the BRET method may be performed by publicly known methods, for example, by modifications of the methods described in Biochem. J. 2005, 385, 625-637, Expert Opin. Ther Tarets, 2007 11: 541-556, etc. Assay for the enzyme activity may also be performed by publicly known methods, for example, by modifications of the methods described in Nat Methods 2006, 3:165-174, Biotechnol J. 2008, 3:311-324, etc.

Furthermore, the present invention provides a kit used for the method of analysis described above. The kit comprises coelenterazine analog of the present invention, the luminescent catalyst, and the organic compound and/or fluorescent protein. Alternatively, the kit comprises the photoprotein of the present invention and the organic compound and/or fluorescent protein. Reagents including coelenterazine analog, the luminescent catalyst, the photoprotein of the invention, the organic compound, the fluorescent protein, etc. may be dissolved in a suitable solvent and prepared into a form suitable for storage. At least one selected from the group consisting of water, ethanol, various buffer solutions and the like may be used as the solvent. The kit may additionally contain, if necessary, at least one selected from the group consisting of exclusive containers, other necessary accessories, instruction manuals, etc.

All literatures and publications mentioned in this specification are herein incorporated in their entirety by reference into the specification, irrespective of their purposes. The specification includes all of the contents as disclosed in the claims, specification and drawings of Japanese Patent Application No. 2009-27921 (filed Feb. 9, 2009), based on which the priority of the present application is enjoyed.

The objects, characteristics, and advantages of the present invention as well as the idea thereof are apparent to those skilled in the art from the descriptions given herein, and those skilled in the art can easily implement the present invention. It is to be understood that the best mode to carry out the invention and specific examples are to be taken as preferred embodiments of the present invention. These descriptions are only for illustrative and explanatory purposes and are not intended to restrict the invention thereto. It is further apparent to those skilled in the art that various modifications may be made based on the descriptions given herein within the intent and scope of the present invention disclosed herein.

In the following EXAMPLES, the ratios of solvent mixtures for chromatography are all by v/v unless otherwise indicated.

EXAMPLES Synthesis Examples Outline of Synthesis of Coelenterazine Analog (CTZ Analog)

The outline of synthesis for i-coelenterazine (i-CTZ (R¹=I, R²=H, R³=H)), n-coelenterazine (n-CTZ (R¹, R²=benzo, R³=H)), me-coelenterazine (me-CTZ (R¹=CH₃, R²=H, R³=H)), et-coelenterazine (et-CTZ (R¹=C₂H₅, R²=H, R³=H)), cf3-coelenterazine (cf3-CTZ (R¹=CF₃, R²=H, R³=H), meo-coelenterazine (meo-CTZ (R¹=OCH₃, R²=H, R³=H)), 3me-coelenterazine (3me-CTZ (R¹=H, R²═CH₃, R³=H)), 3meo-coelenterazine (3meo-CTZ (R¹=H, R²=OCH₃, R³=H)), αmeh-coelenterazine (αmeh-CTZ (R¹=H, R²=H, R³=CH₃)) and 3-isocoelenterazine (3iso-CTZ (R¹=H, R²=OH, R³=H)) using a keto acetal is as shown below.

The outline of synthesis for i-coelenterazine (i-CTZ) using a ketoaldehyde is as shown below.

Process

In the synthesis of CTZ analogs, flash column chromatography was performed using silica gel (37563-85 manufactured by Kanto Chemical Co., Inc., Silica Gel 60N (spherical, neutral), and particle size of 40-50 μm), except that acidic silica gel (37562-79 manufactured by Kanto Chemical Co., Inc., Silica Gel 60 (spherical), and particle size of 40-50 μm) was used for the purification of CTZ analogs.

Thin layer chromatography (TLC) was performed using a glass plate (1.05715 manufactured by MERCK Inc., Silica Gel 60 F₂₄) precoated with silica gel.

The purity of CTZ analogs was confirmed using high performance liquid chromatography (HPLC): 1100 Series HPLC System manufactured by Agilent Inc.: measurement conditions, column: Lichrosorb (registered trademark) RP-18 (5 μm, 4.0 mm i.d.×125 mm, manufactured by Merck Chemicals); moving phase: gradient 60-100% methanol/0.1% aqueous TFA for 40 min; flow rate: 0.45 mL/min; detection: UV 225 nm; volume of injection sample: 0.5 mg/5 mL in methanol/0.1% aqueous TFA=6/4.

Melting point (Mp) was measured on a micro melting point determination apparatus MP-J3 manufactured by YANACO, Inc. (uncorrected data)

Ultraviolet absorption spectra (UV) of CTZ analogs (20 μM methanol solution) were measured at 25° C. in quartz cells (optical path length of 10 mm) with UV-3100 UV-Visible-Near-Infrared Spectrophotometer manufactured by SHIMADZU Corporation under the conditions of high speed at scan speed.

Fluorescence spectra (FL) of CTZ analogs (8 μg/mL methanol) were measured at 25° C. in quartz cells (optical path length of 10 mm) with FP-6500 spectrofluorimeter manufactured by JASCO, under the conditions for excitation wavelength of 330 nm, excitation side bandwidth of 3 nm, fluorescence emission side of 3 nm, response of 0.5 second, sensitivity of medium and scan speed of 100 nm/min.

¹H NMR spectra were obtained with a Unity Plus 400 nuclear magnetic resonance spectrometer manufactured by Varian Corp. ¹³C NMR spectra were obtained with a JNM-EX270 nuclear magnetic resonance spectrometer manufactured by JEOL Co., Ltd. ¹⁹F NMR spectra were obtained with a Mercury 300 spectrometer manufactured by Varian Corp. CDCl₃ or CD₃OD (both manufactured by CIL Inc.) was used as a solvent for the measurement of NMR spectra.

Chemical shifts (δ) were expressed in terms of relative values using, as an internal standard, tetramethylsilane ((CH₃)₄Si) (measurements of ¹H NMR in CDCl₃; δ 0 ppm), the peak derived from non-deuterated solvent for measurements (measurements of ¹H NMR in CD₃OD; δ 3.31 ppm, measurements of ¹³C NMR in CDCl₃; δ 77.0 ppm) or hexafluorobenzene (measurements of ¹⁹F NMR; δ 0 ppm). The binding constant (J) was shown by Hz. Abbreviations s, d, t, q, m and br denote singlet, doublet, triplet, quartet, multiplet, and broad, respectively.

Infrared spectroscopic spectra (IR) were measured by diffuse reflectance method on an IRPrestige-21 Fourier Transform Infrared Spectrophotometer manufactured by SHIMADZU Corporation, equipped with a DRS-8000A diffuse reflectance measuring device.

High resolution mass spectrometric spectra (HRMS) were measured on JMS-700 manufactured by JEOL by the electron impact ionization (EI⁺) method, or the fast atom bombardment (FAB⁺) method. In the FAB⁺ method, m-nitrobenzyl alcohol (NBA) or glycerol was used as a matrix.

Synthesis Example 1 Synthesis of i-coelenterazine (i-CTZ)

Synthesis Example 1-1

Under an argon atmosphere, to 4-iodophenylacetic acid (11) (prepared by the process described in Chen, Q.-H. et al., Bioorg. Med. Chem. 14, 7898-7909 (2006)) (1.06 g, 4.05 mmol) was added thionyl chloride (5.00 mL, 68.6 mmol) and heated to reflux (100° C.) for 1.5 h. After cooling to room temperature, the mixture was concentrated under reduced pressure to give 4-iodophenylacetyl chloride (12) as a brown oily crude product, which was used in the next reaction without further purification.

Synthesis Example 1-2

Under an argon atmosphere, 4-iodophenylacetyl chloride (12) prepared above was dissolved in tetrahydrofuran (THF) (2 mL) and acetonitrile (2 mL) and cooled to 0° C. To this was slowly added a solution of trimethylsilyldiazomethane in diethyl ether (2.0 M, 4.00 mL, 8.00 mmol), which was stirred overnight (14 h) after warming up to room temperature. After concentrating under reduced pressure, the residue was purified by silica gel flash column chromatography (n-hexane/diethyl ether=1/1) to give 1-diazo-3-(4-iodophenyl)propan-2-one (13) as a pale yellow solid (635 mg, 2.22 mmol, 54.9%, 2 steps).

TLC R_(f)=0.34 (n-hexane/diethyl ether=1/2);

¹H NMR (400 MHz, CDCl₃) δ 3.55 (s, 2H), 5.14 (s, 1H), 6.97-7.01 (AA′BB′, 2H), 7.65-7.69 (AA′BB′, 2H);

¹³C NMR (67.8 MHz, CDCl₃) δ 47.3, 55.1, 92.9, 131.4 (2C), 134.2, 137.9 (2C), 191.9;

IR (KBr, cm⁻¹) 737, 802, 841, 1007, 1138, 1306, 1371, 1402, 1483, 1630, 2102, 2114, 3076;

HRMS (EI) m/z 285.9597 (M, C₉H₇IN₂O required 285.9603).

Synthesis Example 1-3

Under an argon atmosphere, 1-diazo-3-(4-iodophenyl)propan-2-one (13) (1.11 g, 3.88 mmol) was dissolved in anhydrous ethanol (10 mL) and cooled to 0° C. To this was added tert-butyl hypochlorite (440 μL, 3.89 mmol) and stirred for an hour at the same temperature. After concentrating under reduced pressure, the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=9/1) to give 1,1-diethoxy-3-(4-iodophenyl)propan-2-one (14) as a yellow oily substance (758 mg, 2.18 mmol, 56.1%).

TLC R_(f)=0.27 (n-hexane/ethyl acetate=9/1);

¹H NMR (400 MHz, CDCl₃) δ 1.25 (t, 6H, J=7.0 Hz), 3.55 (dq, 2H, J=9.5, 7.0 Hz), 3.71 (dq, 2H, J=9.5, 7.0 Hz), 3.83 (s, 2H), 4.61 (s, 1H), 6.94-6.98 (AA′BB′, 2H), 7.62-7.66 (AA′BB′, 2H);

¹³C NMR (67.8 MHz, CDCl₃) δ 15.2 (2C), 43.0, 63.6 (2C), 92.5, 102.6, 131.9 (2C), 133.5, 137.6 (2C), 202.7;

IR (KBr, cm⁻¹) 718, 1007, 1061, 1098, 1157, 1315, 1369, 1400, 1443, 1485, 1584, 1643, 1732, 2882, 2928, 2976, 3321;

HRMS (FAB⁺/NBA) m/z 349.0303 (M+H, C₁₃H₁₈IO₃ required 349.0301).

Synthesis Example 1-4

Under an argon atmosphere, 1,1-diethoxy-3-(4-iodophenyl)propan-2-one (14) (421 mg, 1.21 mmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (prepared by the process described in Adamczyk, M. et al., Org. Prep. Proced. Int., 33, 477-485 (2001)) (222 mg, 801 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (15 h) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=20/1→10/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give i-coelenterazine (15, i-CTZ) as an ocher powder (45.2 mg, 84.7 μmol, 10.6%).

TLC R_(f)=0.52 (ethyl acetate/methanol=20/1);

HPLC retention time 21.5 min;

Mp 157-159° C. (dec.);

UV (MeOH) λmax (ε)=259 (24900), 343 (5500), 432 (8300) nm;

FL (MeOH) λmax Em. 545.5 nm;

¹H NMR (400 MHz, CD₃OD) δ 4.18 (s, 2H), 4.48 (s, 2H), 6.88-6.92 (AA′BB′, 2H), 7.09-7.13 (AA′BB′, 2H), 7.22-7.42 (m, 5H), 7.55-7.60 (AA′BB′, 2H), 7.60-7.66 (AA′BB′, 2H), 7.94 (br, ¹H);

IR (KBr, cm⁻¹) 700, 839, 1007, 1171, 1236, 1277, 1506, 1541, 1558, 1609, 1647, 2810, 2934, 2965, 3030, 3059;

HRMS (EI) m/z 533.0587 (M, C₂₆H₂₀IN₃O₂ required 533.0600).

Synthesis Example 2 Synthesis of n-coelenterazine (n-CTZ)

Synthesis Example 2-1

Under an argon atmosphere, to a solution of ethyl diethoxyacetate (900 μL, 5.06 mmol) in THF (15 mL) was added slowly a diethyl ether solution of (2-naphtalenylmethyl)magnesium bromide (21) (0.25 M, 25.0 mL, 6.25 mmol) at −78° C. After stirring at for 2.5 h at −78° C., the mixture was gradually warmed to room temperature and stirred for 15 h. To this was added 20% aqueous solution of ammonium chloride (10 mL) and the product was extracted with ethyl acetate (×3). The organic layer was sequentially washed with water (×1) and saturated brine (×1), and dried over anhydrous sodium sulfate. After filtration, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=19/1) to give 1,1-diethoxy-3-(2-naphthalenyl)propan-2-one (22) as a colorless oily substance (675 mg, 2.48 mmol, 49.0%).

TLC R_(f)=0.37 (n-hexane/ethyl acetate=9/1);

¹H NMR (400 MHz, CDCl₃) δ 1.26 (t, 6H, J=7.0 Hz), 3.56 (dq, 2H, J=9.5, 7.0 Hz), 3.72 (dq, 2H, J=9.5, 7.0 Hz), 4.06 (s, 2H), 4.66 (s, 1H), 7.35 (dd, 1H, J=1.8, 8.5 Hz), 7.42-7.49 (m, 2H), 7.69 (s, 1H), 7.76-7.84 (m, 3H);

¹³C NMR (67.8 MHz, CDCl₃) δ 15.2 (2C), 43.9, 63.5 (2C), 102.4, 125.7, 126.1, 127.66, 127.69, 128.0, 128.1, 128.5, 131.4, 132.4, 133.5, 203.2;

IR (KBr, cm⁻¹) 812, 1017, 1061, 1098, 1125, 1310, 1370, 1508, 1732, 2882, 2928, 2976, 3053;

HRMS (FAB⁺/NBA+NaI) m/z 295.1316 (M+Na, C₁₇H₂₀O₃Na required 295.1310).

Synthesis Example 2-2

Under an argon atmosphere, 1,1-diethoxy-3-(2-naphthalenyl)propan-2-one (22) (369 mg, 1.35 mmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (310 mg, 1.12 mmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred at 100° C. overnight (15 hours). After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=50/1→20/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give n-coelenterazine (23, n-CTZ) as a yellow powder (88.5 mg, 193 μmol, 17.3%).

TLC R_(f)=0.47 (ethyl acetate/methanol=20/1);

HPLC retention time 20.6 min;

Mp 149-151° C. (dec.);

UV (MeOH) λ_(max) (ε)=262.5 (34500), 349.5 (6400), 432.5 (11700) nm;

FL (MeOH)) λ_(max) Em. 434.5, 544 nm;

¹H NMR (400 MHz, CD₃OD) δ 4.35 (s, 2H), 4.41 (s, 2H), 6.85-6.91 (AA′BB′, 2H), 7.18-7.31 (m, 3H), 7.35-7.45 (m, 4H), 7.46-7.54 (m, 3H), 7.68-7.80 (m, 5H);

IR (KBr, cm⁻¹) 698, 760, 815, 839, 1173, 1242, 1277, 1456, 1508, 1541, 1558, 1611, 2812, 2967, 3055, 3152;

HRMS (EI) m/z 457.1778 (M, C₃₀H₂₃N₃O₂ required 457.1790).

Synthesis Example 3 Synthesis of me-coelenterazine (me-CTZ)

Synthesis Example 3-1

The magnesium turnings (270 mg, 11.1 mmol) were dried in vacuo by heating with a heat gun. After cooling to room temperature and being placed under an argon atmosphere, THF (8 mL) was added thereto, followed by slow addition of 4-methylbenzyl chloride (31) (1.35 mL, 10.2 mmol) at room temperature. The reaction mixture became warm as the result of an exothermic reaction and most of the magnesium turnings were consumed. After cooling to room temperature, it was used directly in the next reaction as a THF solution of (4-methylbenzyl)magnesium chloride (32).

Under an argon atmosphere, to a solution of ethyl diethoxyacetate (1.80 mL, 10.1 mmol) in THF (20 mL) was added slowly a THF solution of (32) prepared above at −78° C. After stirring at −78° C. for an hour, to this was added 20% aqueous solution of ammonium chloride (10 mL) and the product was extracted with ethyl acetate (×3). The organic layer was sequentially washed with water (×1) and saturated brine (×1), and dried over anhydrous sodium sulfate. After filtration, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=9/1) to give 1,1-diethoxy-3-(4-methylphenyl)propan-2-one (33) as a colorless oily substance (1.96 g, 8.27 mmol, 82.4%).

TLC R_(f)=0.45 (n-hexane/ethyl acetate=9/1);

¹H NMR (400 MHz, CDCl₃) δ 1.25 (t, 6H, J=7.0 Hz), 2.32 (s, 3H), 3.55 (dq, 2H, J=9.5, 7.0 Hz), 3.70 (dq, 2H, J=9.5, 7.0 Hz), 3.85 (s, 2H), 4.63 (s, 1H), 7.08-7.14 (2AA′BB′, 41-H);

¹³C NMR (67.8 MHz, CDCl₃) δ 15.1 (2C), 21.0, 43.3, 63.3 (2C), 102.2, 129.1 (2C), 129.6 (2C), 130.6, 136.3, 203.3;

IR (KBr, cm⁻¹) 772, 804, 851, 1022, 1063, 1098, 1146, 1312, 1516, 1732, 2884, 2926, 2976;

HRMS (EI) m/z 236.1409 (M, C₁₄H₂₀O₃ required 236.1412).

Synthesis Example 3-2

Under an argon atmosphere, 1,1-diethoxy-3-(4-methylphenyl)propan-2-one (33) (216 mg, 914 μmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (196 mg, 707 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (14 hours) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=50/1→20/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give me-coelenterazine (34, me-CTZ) as a yellow powder (180 mg, 427 μmol, 60.4%).

TLC R_(f)=0.51 (ethyl acetate/methanol=20/1);

HPLC retention time 17.1 min;

Mp 150-152° C. (dec.);

UV (MeOH) λ_(max) (ε)=260 (20900), 351 (4600), 437.5 (8600) nm;

FL (MeOH) λ_(max) Em. 433.5, 545 nm;

¹H NMR (400 MHz, CD₃OD) δ2.29 (s, 3H), 4.22 (s, 2H), 4.52 (s, 2H), 6.88-6.93 (AA′BB′, 2H), 7.09-7.14 (AA′BB′, 2H), 7.15-7.20 (AA′BB′, 2H), 7.22-7.34 (m, 3H), 7.38-7.42 (m, 2H), 7.67-7.73 (AA′BB′, 2H), 8.23 (br, ¹H);

IR (KBr, cm⁻¹) 700, 820, 843, 1171, 1238, 1279, 1508, 1541, 1589, 1609, 2812, 2924, 3028;

HRMS (FAB⁺/glycerol) m/z 422.1880 (M+H, C₂₇H₂₄N₃O₂ required 422.1869).

SYNTHESIS EXAMPLE 4 Synthesis of et-coelenterazine (et-CTZ)

Synthesis Example 4-1

The magnesium turnings (273 mg, 11.2 mmol) were dried in vacuo by heating with a heat gun. After cooling to room temperature and being placed under an argon atmosphere, THF (8 mL) was added thereto, followed by slow addition of 4-ethylbenzyl chloride (41) (1.48 mL, 9.95 mmol) at room temperature. The reaction mixture became warm as the result of an exothermic reaction and most of magnesium turnings were consumed. After cooling to room temperature, it was used directly in the next reactions as a THE solution of (4-ethylbenzyl)magnesium chloride (42).

Under an argon atmosphere, to a solution of ethyl diethoxyacetate (1.80 mL, 10.1 mmol) in THF (20 mL) was added slowly a THF solution (42) of (4-ethylbenzyl)magnesium chloride prepared above at −78° C. After stirring at −78° C. for an hour, to this was added 20% aqueous solution of ammonium chloride (10 mL), and the product was extracted with ethyl acetate (×3). The organic layer was sequentially washed with water (×1) and saturated brine (×1), and dried over anhydrous sodium sulfate. After filtration, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=9/1) to give 1,1-diethoxy-3-(4-ethylphenyl)propan-2-one (43) as a colorless oily substance (1.29 g, 5.04 mmol, 50.6%).

TLC R_(f)=0.43 (n-hexane/ethyl acetate=9/1)

¹H NMR (400 MHz, CDCl₃) □ δ 1.22 (t, 3H, J=7.6 Hz), 1.25 (t, 6H, J=7.0 Hz), 2.63 (q, 2H, J=7.6 Hz), 3.55 (dq, 2H, J=9.5, 7.0 Hz), 3.70 (dq, 2H, J=9.5, 7.0 Hz), 3.86 (s, 2H), 4.63 (s, 1H), 7.10-7.17 (2AA′BB′, 4H);

¹³C NMR (67.8 MHz, CDCl₃) δ 15.1 (2C), 15.5, 28.4, 43.2, 63.2 (2C), 102.2, 127.9 (2C), 129.6 (2C), 130.8, 142.6, 203.2;

IR (KBr, cm⁻¹) 810, 853, 1022, 1061, 1099, 1151, 1314, 1514, 1732, 2874, 2930, 2974;

HRMS (EI) m/z 250.1566 (M, C₁₅H₂₂O₃ required 250.1569).

Synthesis Example 4-2

Under an argon atmosphere, 1,1-diethoxy-3-(4-ethylphenyl)propan-2-one (43) (301 mg, 1.20 mmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (238 mg, 858 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (17 hours) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=50/1→10/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give et-coelenterazine (44, et-CTZ) as a yellow powder (240 mg, 551 μmol, 64.2%).

TLC R_(f)=0.52 (ethyl acetate/methanol=20/1);

HPLC retention time 20.5 min;

Mp 145-147° C. (dec.);

UV (MeOH) λ_(max) (ε)=259.5 (22300), 342.5 (5000), 434.5 (8800) nm;

FL (MeOH) λ_(max) Em. 433, 546.5 nm;

¹H NMR (400 MHz, CD₃OD) δ 1.19 (t, 3H, J=7.6 Hz), 2.60 (q, 2H, J=7.6 Hz), 4.24 (s, 2H), 4.53 (s, 2H), 6.88-6.96 (AA′BB′, 2H), 7.17-7.22 (2AA′BB′, 4H), 7.24-7.34 (m, 3H), 7.38-7.43 (m, 2H), 7.69-7.75 (AA′BB′, 2H), 8.26 (br, ¹H);

IR (KBr, cm⁻¹) 700, 820, 840, 1171, 1238, 1277, 1508, 1541, 1589, 1609, 1647, 2893, 2930, 2963, 3028;

HRMS (FAB⁺/glycerol) m/z 436.2026 (M+H, C₂₈H₂₆N₃O₂ required 436.2025).

Synthesis Example 5 Synthesis of cf3-coelenterazine (cf3-CTZ)

Synthesis Example 5-1

Under an argon atmosphere, to 4-trifluoromethylphenylacetic acid (51) (817 mg, 4.00 mmol) was added thionyl chloride (5.00 mL, 68.9 mmol), and heated to reflux (100° C.) for 1.5 h. After cooling to room temperature, the mixture was concentrated under reduced pressure to give 4-trifluoromethylphenylacetyl chloride (52) as a brown oily crude product, which was used in the next reaction without further purification.

Synthesis Example 5-2

Under an argon atmosphere, p-trifluoromethylphenylacetyl chloride (52) prepared above was dissolved in THF (2 mL) and acetonitrile (2 mL), and cooled to 0° C. To this was slowly added a solution of trimethylsilyldiazomethane in diethyl ether (2.0 M, 4.00 mL, 8.00 mmol), which was stirred overnight (20 hours) after warming up to room temperature. After being concentrated under reduced pressure, the residue was purified by silica gel flash column chromatography (n-hexane/diethyl ether=1/1) to give 1-diazo-3-[4-(trifluoromethyl)phenyl]propan-2-one (53) as a pale yellow oily substance (666 mg, 2.92 mmol, 72.9%, 2 steps).

TLC R_(f)=0.31 (n-hexane/diethyl ether=1/2);

¹H NMR (400 MHz, CDCl₃) δ 3.68 (s, 2H), 5.18 (s, 1H), 7.34-7.40 (AA′BB′, 2H), 7.59-7.63 (AA′BB′, 2H);

¹³C NMR (67.8 MHz, CDCl₃) δ 47.4, 55.3, 124.2 (q, ¹J_(C-F)=271.9 Hz), 125.7 (2C, q, ³J_(C-F)=3.8 Hz), 129.1 (q, ²J_(C-F)=32.4 Hz), 129.8 (2C), 138.6, 191.4;

¹⁹F NMR (282 MHz, CDCl₃) δ 99.3 (s);

IR (KBr, cm⁻¹) 743, 824, 854, 1020, 1067, 1119, 1164, 1325, 1366, 1420, 1620, 1634, 1639, 2107, 3086;

HRMS (EI) m/z 228.0511 (M, C₁₀H₇F₃N₂O required 228.0510).

Synthesis Example 5-3

Under an argon atmosphere, 1-diazo-3-[4-(trifluoromethyl)phenyl]propan-2-one (53) (661 mg, 2.90 mmol) was dissolved in anhydrous ethanol (6 mL) and cooled to 0° C. To this was added tert-butyl hypochlorite (330 μL, 2.92 mmol) and stirred for an hour at the same temperature. After being concentrated under reduced pressure, the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=18/1) to give 1,1-diethoxy-3-[4-(trifluoromethyl)phenyl]propan-2-one (54) as a colorless oily substance (544 mg, 1.87 mmol, 64.7%).

TLC R_(f)=0.29 (n-hexane/ethyl acetate=9/1);

¹H NMR (400 MHz, CDCl₃) δ 1.26 (t, 6H, J=7.0 Hz), 3.57 (dq, 2H, J=9.2, 7.0 Hz), 3.74 (dq, 2H, J=9.2, 7.0 Hz), 3.96 (s, 2H), 4.62 (s, 1H), 7.30-7.35 (AA′BB′, 2H), 7.54-7.60 (AA′BB′, 2H);

¹³C NMR (67.8 MHz, CDCl₃) δ 15.2 (2C), 43.0, 63.7 (2C), 102.8, 124.3 (q, ¹J_(C-F)=271.9 Hz), 125.3 (2C, q, ³J_(C-F)=3.8 Hz), 129.6 (q, ²J_(C-F)=32.4 Hz), 130.3 (2C), 138.1, 202.4;

¹⁹F NMR (282 MHz, CDCl₃) δ 99.3 (s);

IR (KBr, cm⁻¹) 864, 1020, 1067, 1109, 1125, 1165, 1325, 1732, 2884, 2934, 2980;

HRMS (FAB⁺/NBA+NaI) m/z 313.1031 (M+Na, C₁₄H₁₇F₃O₃Na required 313.1027).

Synthesis Example 5-4

Under an argon atmosphere, 1,1-diethoxy-3-[4-(trifluoromethyl)phenyl]propan-2-one (54) (359 mg, 1.24 mmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (217 mg, 782 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (17 hours) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=20/1→10/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give cf3-coelenterazine (55, cf3-CTZ) as a yellow powder (232 mg, 488 μmol, 62.4%).

TLC R_(f)=0.30 (ethyl acetate/methanol=20/1);

HPLC retention time 19.9 min;

Mp 157-161° C. (dec.);

UV (MeOH) λ_(max) (ε)=259 (27000), 341.5 (5600), 440.5 (10500) nm;

FL (MCOH) λ_(max) Em. 549.5 nm;

¹H NMR (400 MHz, CD₃OD) δ 4.37 (s, 2H), 4.55 (s, 2H), 6.90-6.95 (AA′BB′, 2H), 7.23-7.34 (m, 3H), 7.39-7.43 (m, 2H), 7.49-7.53 (AA′BB′, 2H), 7.61-7.71 (2AA′BB′, 4H), 8.23 (br, ¹H);

¹⁹F NMR (282 MHz, CDCl₃) δ 101.8 (s);

IR (KBr, cm⁻¹) 702, 818, 1018, 1067, 1121, 1177, 1327, 1508, 1541, 1593, 1609, 1655, 2814, 2862, 2928, 3032, 3256;

HRMS (FAB⁺/glycerol) m/z 476.1580 (M+H, C₂₇H₂₁F₃N₃O₂ required 476.1586).

Synthesis Example 6 Synthesis of Meo-Coelenterazine (meo-CTZ)

Synthesis Example 6-1

Under an argon atmosphere, 4-methoxyphenylacetyl chloride (61) (952 mg, 5.16 mmol) was dissolved in THF (2.5 mL) and acetonitrile (2.5 mL) and cooled to 0° C. To this was slowly added a solution of trimethylsilyldiazomethane in diethyl ether (2.0 M, 5.00 mL, 10.0 mmol), which was stirred overnight (15 hours) after warming up to room temperature. After concentrating under reduced pressure, the residue was purified by silica gel flash column chromatography (n-hexane/diethyl ether=1/1) to give 1-diazo-3-(4-methoxyphenyl)propan-2-one (62) as a pale yellow oily substance (692 mg, 3.64 mmol, 70.6%).

TLC R_(f)=0.41 (n-hexane/diethyl ether=1/2);

¹H NMR (400 MHz, CDCl₃) δ 3.56 (s, 2H), 3.81 (s, 3H), 5.11 (s, 1H), 6.85-6.90 (AA′BB′, 2H), 7.12-7.18 (AA′BB′, 2H);

¹³C NMR (67.8 MHz, CDCl₃) δ 47.3, 54.7, 55.3, 114.3 (2C), 126.6, 130.5 (2C), 158.9, 193.4;

IR (KBr, cm⁻¹) 821, 851, 943, 1032, 1117, 1179, 1248, 1358, 1512, 1611, 1632, 2102, 2835, 2907, 2934, 3098, 3530;

HRMS (EI) m/z 190.0743 (M, C₁₀H₁₀N₂O₂ required 190.0742).

Synthesis Example 6-2

Under an argon atmosphere, 1-diazo-3-(4-methoxyphenyl)propan-2-one (62) (477 mg, 2.51 mmol) was dissolved in anhydrous ethanol (5 mL) and cooled to 0° C. To this was added tert-butyl hypochloride (285 μL, 2.52 mmol) and stirred for an hour at the same temperature. After concentrating under reduced pressure, the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=10/1) to give 1,1-diethoxy-3-(4-methoxyphenyl)propan-2-one (63) as a colorless oily substance (357 mg, 1.41 mmol, 56.4%).

TLC R_(f)=0.29 (n-hexane/ethyl acetate=9/1);

¹H NMR (400 MHz, CDCl₃) δ 1.25 (t, 6H, J=7.0 Hz), 3.55 (dq, 2H, J=9.5, 7.0 Hz), 3.70 (dq, 2H, J=9.5, 7.0 Hz), 3.79 (s, 3H), 3.83 (s, 2H), 4.63 (s, 1H), 6.83-6.91 (AA′BB′, 2H), 7.10-7.17 (AA′BB′, 2H);

¹³C NMR (67.8 MHz, CDCl₃) δ 15.2 (2C), 42.9, 55.3, 63.4 (2C), 102.3, 114.0 (2C), 125.8, 130.8 (2C), 158.6, 203.6;

IR (KBr, cm⁻¹) 1036, 1063, 1098, 1177, 1512, 1612, 1732, 2835, 2897, 2933, 2976;

HRMS (EI) m/z 252.1360 (M, C₁₄H₂₀O₄ required 252.1362).

Synthesis Example 6-3

Under an argon atmosphere, 1,1-diethoxy-3-(4-methoxyphenyl)propan-2-one (63) (355 mg, 1.41 mmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (221 mg, 797 mol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (17 hours) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=20/1→10/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give meo-coelenterazine (64, meo-CTZ) as an ocher powder (174 mg, 398 mmol, 49.9%).

TLC R_(f)=0.31 (ethyl acetate/methanol=20/1);

HPLC retention time 13.7 min;

Mp 137-139° C. (dec.);

UV (MeOH) λ_(max) (ε)=267 (26000), 344.5 (7400), 435 (8500) nm;

FL (MeOH) λ_(max) Em. 435.5, 549 nm;

¹H NMR (400 MHz, CD₃OD) δ 3.75 (s, 3H), 4.21 (s, 2H), 4.54 (s, 2H), 6.84-6.94 (2AA′BB′, 4H), 7.18-7.36 (m, 5H), 7.38-7.43 (m, 2H), 7.69-7.75 (AA′BB′, 2H), 8.31 (br, ¹H);

IR (KBr, cm⁻¹) 820, 839, 1177, 1248, 1508, 1558, 1585, 1609, 1647, 2835, 2951, 3030, 3063;

HRMS (FAB⁺/glycerol) m/z 438.1814 (M+H, C₂₇H₂₄N₃O₃ required 438.1818).

Synthesis Example 7 Synthesis of 3me-coelenterazine (3me-CTZ)

Synthesis Example 7-1

The magnesium turnings (271 mg, 11.1 mmol) were dried in vacuo by heating with a heat gun. After cooling to room temperature and being placed under an argon atmosphere, THF (8 mL) was added thereto, followed by slow addition of 3-methylbenzyl chloride (71) (1.35 mL, 10.2 mmol) at room temperature. The reaction mixture became warm as the result of an exothermic reaction and most of the magnesium turnings were consumed. After cooling to room temperature, it was used directly in the next reaction as a THF solution of (3-methylbenzyl)magnesium chloride (72).

Under an argon atmosphere, to a solution of ethyl diethoxyacetate (1.80 mL, 10.1 mmol) in THF (20 mL) was added slowly a THF solution of (72) prepared above at −78° C. After stirring at −78° C. for an hour, to this was added 20% aqueous solution of ammonium chloride (10 mL) and the product was extracted with ethyl acetate (×3). The organic layer was sequentially washed with water (×1) and saturated brine (×1), and dried over anhydrous sodium sulfate. After filtration, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=9/1) to give 1,1-diethoxy-3-(3-methylphenyl)propan-2-one (73) as a colorless oily substance (1.38 g, 5.84 mmol, 58.0%).

TLC R_(f)=0.40 (n-hexane/ethyl acetate=9/1);

¹H NMR (400 MHz, CDCl₃) δ 1.25 (t, 6H, J=7.0 Hz), 2.33 (s, 3H), 3.55 (dq, 2H, J=9.5,

7.0 Hz), 3.70 (dq, 2H, J=9.5, 7.0 Hz), 3.85 (s, 2H), 4.63 (s, 1H), 6.99-7.08 (m, 3H), 7.18-7.23 (m, 1H);

¹³C NMR (67.8 MHz, CDC₃) δ 15.2 (2C), 21.4, 43.7, 63.4 (2C), 102.3, 126.8, 127.6, 128.4, 130.6, 133.7, 138.1, 203.3;

IR (KBr, cm⁻¹) 700, 758, 1063, 1099, 1157, 1314, 1736, 2880, 2928, 2976;

HRMS (FAB⁺/NBA+NaI) m/z 259.1307 (M+Na, C₁₄H₂₀O₃Na required 259.1310).

Synthesis Example 7-2

Under an argon atmosphere, 1,1-diethoxy-3-(3-methylphenyl)propan-2-one (73) (265 mg, 1.12 mmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (202 mg, 728 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (17 hours) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=50/1→20/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give 3me-coelenterazine (74, 3me-CTZ) as a yellow powder (148 mg, 351 mmol, 48.2%).

TLC R_(f)=0.52 (ethyl acetate/methanol=20/1);

HPLC retention time 17.1 min;

Mp 142-145° C. (dec.);

UV (MeOH) λ_(max) (ε)=260 (22500), 349.5 (5100), 435 (8900) nm;

FL (MeOH) λ_(max) Em. 437, 547 nm;

¹H NMR (400 MHz, CD₃OD) δ 2.29 (s, 3H). 4.24 (s, 2H), 4.54 (s, 2H), 6.88-6.94 (AA′BB′, 2H), 7.03-7.14 (m, 3H), 7.16-7.21 (m, 1H), 7.22-7.34 (m, 3H), 7.39-7.43 (m, 2H), 7.69-7.77 (AA′BB′, 2H), 8.30 (br, ¹H);

IR (KBr, cm⁻¹) 700, 748, 820, 841, 1171, 1236, 1277, 1506, 1541, 1589, 1608, 1647, 2862, 2922, 3028;

HRMS (FAB⁺/glycerol) m/z 422.1863 (M+H, C₂₇H₂₄N₃O₂ required 422.1869).

Synthesis Example 8 Synthesis of 3meo-coelenterazine (3meo-CTZ)

Synthesis Example 8-1

Under an argon atmosphere, 3-methoxyphenylacetyl chloride (81) (952 mg, 5.16 mmol) was dissolved in THF (2.5 mL) and acetonitrile (2.5 mL) and cooled to 0° C. To this was slowly added a solution of trimethylsilyldiazomethane in diethyl ether (2.0 M, 5.00 mL, 10.0 mmol), which was stirred overnight (15 hours) after warming up to room temperature. After concentrating under reduced pressure, the residue was purified by silica gel flash column chromatography (n-hexane/diethyl ether=3/2) to give 1-diazo-3-(3-methoxyphenyl)propan-2-one (82) as a pale yellow oily substance (650 mg, 3.42 mmol, 66.3%).

TLC R_(f)=0.38 (n-hexane/diethyl ether=1/2);

¹H NMR (400 MHz, CDCl₃) δ 3.59 (s, 2H), 3.81 (s, 3H), 5.14 (s, 1H), 6.76-6.86 (m, 3H), 7.23-7.29 (m, 1H);

¹³C NMR (67.8 MHz, CDCl₃) δ 47.9, 54.7, 55.0, 112.6, 114.9, 121.6, 129.7, 135.9, 159.8, 192.6;

IR (KBr, cm⁻¹) 691, 764, 876, 949, 1047, 1076, 1150, 1258, 1358, 1489, 1584, 1631, 2104, 2835, 2940, 3003, 3580;

HRMS (EI) m/z 190.0740 (M, C₁₀H₁₀N₂O₂ required 190.0742).

Synthesis Example 8-2

Under an argon atmosphere, 1-diazo-3-(3-methoxyphenyl)propan-2-one (82) (501 mg, 2.63 mmol) was dissolved in anhydrous ethanol (5 mL) and cooled to −18C. To this was added tert-butyl hypochlorite (300 μL, 2.65 mmol) and stirred for an hour at the same temperature. After concentrating under reduced pressure, the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=10/1) to give 1,1-diethoxy-3-(3-methoxyphenyl)propan-2-one (83) as a pale yellow oily substance (371 mg, 1.47 mmol, 55.8%).

TLC R_(f)=0.27 (n-hexane/ethyl acetate=9/1);

¹H NMR (400 MHz, CDCl₃) δ 1.25 (t, 6H, J=7.0 Hz), 3.55 (dq, 2H, J=9.5, 7.0 Hz), 3.70 (dq, 2H, J=9.5, 7.0 Hz), 3.79 (s, 3H), 3.86 (s, 2H), 4.64 (s, 1H), 6.74-6.83 (m, 3H), 7.21-7.25 (m, 1H);

¹³C NMR (67.8 MHz, CDCl₃) δ 15.2 (2C), 43.8, 55.2, 63.4 (2C), 102.3, 112.5, 115.4, 122.2, 129.4, 135.2, 159.7, 203.1;

IR (KBr, cm⁻¹) 1057, 1098, 1150, 1260, 1585, 1732, 2835, 2886, 2934, 2976;

HRMS (FAB⁺/NBA+KCl) m/z 291.0992 (M+K, C₁₄H₂₀O₄K required 291.0999).

Synthesis Example 8-3

Under an argon atmosphere, 1,1-diethoxy-3-(3-methoxyphenyl)propan-2-one (83) (254 mg, 1.01 mmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (209 mg, 754 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (15 hours) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=20/1→10/1) (×2). The solid obtained was further reprecipitated (n-hexane/acetone) to give 3meo-coelenterazine (84, 3meo-CTZ) as an ocher powder (98.7 mg, 226 μmol, 29.9%).

TLC R_(f)=0.33 (ethyl acetate/methanol=20/1);

HPLC retention time 13.8 min;

Mp 136-140° C. (dec.);

UV (MeOH) λ_(max) (ε)=262 (25600), 346.5 (5400), 438.5 (9900) nm;

FL (MeOH) λ_(max) Em. 428.5, 543.5 nm;

¹H NMR (400 MHz, CD₃OD) δ 3.75 (s, 3H), 4.25 (s, 2H), 4.55 (s, 2H), 6.77 (dd, 1H, J=1.9, 8.0 Hz), 6.84-6.92 (m, 4H), 7.16-7.34 (m, 4H), 7.38-7.44 (m, 2H), 7.63-7.70 (AA′BB′, 2H), 8.17 (br, ¹H);

IR (KBr, cm⁻¹) 700, 750, 839, 1045, 1170, 1260, 1506, 1541, 1595, 1608, 1647, 2835, 2938, 3030, 3059;

HRMS (FAB⁺/glycerol) m/z 438.1814 (M+H, C₂₇H₂₄N₃O₃ required 438.1818).

Synthesis Example 9 Synthesis of αmeh-coelenterazine (αmeh-CTZ)

Synthesis Example 9-1

The magnesium turnings (274 mg, 11.3 mmol) were dried in vacuo by heating with a heat gun. After cooling to room temperature and being placed under an argon atmosphere, THF (8 mL) was added thereto, followed by slow addition of 1-chloroethylbenzene (91) (1.35 mL, 10.2 mmol) at room temperature. The reaction mixture became warm as the result of an exothermic reaction and most of magnesium turnings were consumed. After cooling to room temperature, it was used directly in the next reaction as a THF solution of (1-phenylethyl)magnesium chloride (92).

Under an argon atmosphere, to a solution of ethyl diethoxyacetate (1.80 mL, 10.1 mmol) in THF (20 mL) was added slowly a THF solution of (92) prepared above at −78° C. After stirring at −78° C. for 3 hours, to this was added 20% aqueous solution of ammonium chloride (10 mL) and the product was extracted with ethyl acetate (×3). The organic layer was sequentially washed with water (×1) and saturated brine (×1), and dried over anhydrous sodium sulfate. After filtration, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=19/1) to give 1,1-diethoxy-3-phenylbutan-2-one (93) as a colorless oily substance (330 mg, 1.37 mmol, 13.7%).

TLC R_(f)=0.43 (n-hexane/ethyl acetate=9/1);

¹H NMR (400 MHz, CDCl₃) □ δ 1.13 (t, 3H, J=7.0 Hz), 1.20 (t, 3H, J=7.0 Hz), 1.41 (d, 3H, J=7.0 Hz), 3.32 (dq, 1H, J=9.5, 7.0 Hz), 3.47-3.64 (m, 3H), 4.26 (q, 1H, J=7.0 Hz), 4.59 (s, 1H), 7.22-7.36 (m, 5H);

¹³C NMR (67.8 MHz, CDCl₃) δ 15.1, 15.2, 18.1, 47.3, 62.8, 63.0, 101.2, 127.1, 128.3 (2C), 128.7 (2C), 140.2, 205.8;

IR (KBr, cm⁻¹) 700, 1030, 1063, 1103, 1163, 1452, 1493, 1730, 2874, 2932, 2976;

HRMS (FAB⁺/NBA+NaI) m/z 237.1496 (M+H, C₁₄H₂₁O₃ required 237.1491).

Synthesis Example 9-2

Under an argon atmosphere, 1,1-diethoxy-3-phenylbutan-2-one (93) (188 mg, 796 μmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (170 mg, 613 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was further added thereto, and then stirred overnight (16 hours) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=20/1).

The solid obtained was further reprecipitated (n-hexane/acetone) to give αmeh-coelenterazine (94, αmeh-CTZ) as a yellow powder (76.5 mg, 181 μmol, 29.6%).

TLC R_(f)=0.53 (ethyl acetate/methanol=20/1);

HPLC retention time 17.6 min;

Mp 143-145° C. (dec.);

UV (MeOH) λ_(max) (ε)=259.5 (18800), 350 (4400), 439 (7800) nm;

FL (MeOH) λ_(max) Em. 429.5, 551 nm;

¹H NMR (400 MHz, CD₃OD) δ 1.81 (d, 3H, J=7.3 Hz), 4.56 (q, 1H, J=7.3 Hz), 4.58 (s, 2H), 6.89-6.94 (AA′BB′, 2H), 7.18-7.40 (m, 8H), 7.43-7.47 (m, 2H), 7.58-7.65 (AA′BB′, 2H), 8.07 (br, ¹H);

IR (KBr, cm⁻¹) 700, 820, 841, 1177, 1215, 1277, 1454, 1508, 1541, 1558, 1610, 1647, 2876, 2934, 2972, 3030, 3059;

HRMS (FAB⁺/glycerol) m/z 422.1872 (M+H, C₂₇H₂₄N₃O₂ required 422.1869).

Synthesis Example 10 Synthesis of 3-isocoelenterazine (3iso-CTZ)

Synthesis Example 10-1

3-(tert-Butyldimethylsilyloxy)benzyl alcohol (101) (prepared by the process described in Wu, Y.-C. et al., J. Am. Chem. Soc., 130, 7148-7152 (2008)) (7.94 g, 31.4 mmol) was dissolved in dichloromethane (150 mL), and cooled to 0° C. To this were sequentially added triethylamine (9.10 mL, 66.7 mmol) and methanesulfonyl chloride (3.80 mL, 49.1 mmol) and stirred for 25 hours after warming up to room temperature. To this was added water, and the product was extracted with dichloromethane (×3). The organic layer was sequentially washed with water (×1) and saturated brine (×1), and dried over anhydrous sodium sulfate. After filtration, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=95/1) to give 3-(tert-butyldimethylsilyloxy)benzyl chloride (102) as a colorless oily substance (5.90 g, 23.0 mmol, 73.1%).

TLC R_(f)=0.45 (n-hexane);

¹H NMR (400 MHz, CDCl₃) δ 0.20 (s, 6H), 0.99 (s, 9H), 4.53 (s, 2H), 6.79 (dd, 1H, J=1.4, 8.1 Hz), 6.87 (d, 1H, J=1.4 Hz), 6.97 (d, 1H, J=8.1 Hz), 7.20 (dd, 1H, J=8.1, 8.1 Hz).

Synthesis Example 10-2

The magnesium turnings (270 mg, 11.1 mmol) were dried in vacuo by heating with a heat gun. After cooling to room temperature and being placed under an argon atmosphere, THF (17.5 mL) was added thereto, followed by slow addition of 3-(tert-Butyldimethylsilyloxy)benzyl chloride (102) (2.57 g, 10.0 mmol) at room temperature. The reaction mixture was warmed as the result of an exothermic reaction and most of the magnesium turnings were consumed. After cooling to room temperature, it was used directly in the next reaction as a TI-IF solution of 3-(tert-butyldimethylsilyloxy)benzylmagnesium chloride (103) was cooled to room temperature and was used as it was in the next reaction.

Under an argon atmosphere, to a solution of ethyl diethoxyacetate (1.80 mL, 10.1 mmol) in THF (20 mL) was added slowly a THF solution of (103) prepared above at −78° C. After stirring at −78° C. for an hour, to this was added 20% aqueous solution of ammonium chloride (30 mL) and the product was extracted with ethyl acetate (×3). The organic layer was sequentially washed with water (×1) and saturated brine (×1), and dried over anhydrous sodium sulfate. After filtration, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=10/1) to give 3-[3-(tert-butyldimethylsilyloxy]phenyl]-1,1-diethoxypropan-2-one (104) as a colorless oily substance (1.90 g, 5.39 mmol, 53.9%).

TLC R_(f)=0.48 (n-hexanelethyl acetate=10/1);

¹H NMR (400 MHz, CDCl₃) δ 0.19 (s, 6H), 0.97 (s, 9H), 1.25 (t, 6H, J=7.0 Hz), 3.54 (dq, 2H, J=9.5, 7.0 Hz), 3.69 (dq, 2H, J=9.5, 7.0 Hz), 3.82 (s, 2H), 4.63 (s, 1H), 6.68-6.75 (m, 2H), 6.81 (d, 1H, J=7.8 Hz), 7.16 (dd, 1H, J=7.8, 7.8 Hz);

¹³C NMR (67.8 MHz, CDCl₃) δ−4.4 (2C), 15.2 (2C), 18.2, 25.7 (3C), 43.7, 63.3 (2C), 102.2, 118.5, 121.6, 122.8, 129.3, 135.2, 155.7, 202.9;

IR (KBr, cm⁻¹) 781, 839, 982, 1063, 1157, 1275, 1487, 1585, 1601, 1736, 2859, 2886, 2930, 2955, 2974;

HRMS (EI) m/z 352.2073 (M, C₁₉H₃₂O₄Si required 352.2070).

Synthesis Example 10-3

Under an argon atmosphere, 3-[3-(tert-butyldimethylsilyloxy)phenyl]-1,1-diethoxypropan-2-one (104) (390 mg, 1.11 mmol) was dissolved in 1,4-dioxane (2 mL) and water (0.4 mL). To this was added coelenteramine (202 mg, 728 μmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was added thereto, and then stirred overnight (14 hours) at 100° C. After cooling to room temperature, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash chromatography in an argon flow (n-hexane/ethyl acetate=1/2→ethyl acetate→ethyl acetate/methanol=20/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give 3-isocoelenterazine (105, 3 iso-CTZ) as a yellow powder (159 mg, 375 μmol, 51.5%).

TLC R_(f)=0.29 (ethyl acetate/methanol=20/1);

HPLC retention time 8.6 min;

Mp 160-162° C. (dec.);

UV (MeOH) λ_(max) (ε)=265.5 (19300), 351.5 (4400), 433.5 (7700) nm;

FL (MeOH) λ_(max) Em. 429, 549.0 nm;

¹H NMR (400 MHz, CD₃OD) δ 4.20 (s, 2H), 4.53 (s, 2H), 6.66 (dd, 1H, J=1.4, 8.1 Hz), 6.71-6.77 (m, 2H), 6.88-6.93 (AA′BB′, 2H), 7.12 (dd, 1H, J=8.1, 8.1 Hz), 7.22-7.35 (m, 3H), 7.39-7.43 (m, 2H), 7.68-7.75 (AA′BB′, 2H), 8.26 (br, ¹H);

IR (KBr, cm⁻¹) 700, 760, 820, 841, 1171, 1238, 1275, 1456, 1506, 1541, 1591, 1608, 2953, 3063, 3150;

HRMS (FAB⁺/glycerol) m/z 424.1663 (M+H, C₂₆H₂₂N₃O₃ required 424.1661).

Comparative Synthesis Example 1 Synthesis of i-coelenterazine (1-CTZ)

Comparative Synthesis Example 1-1

Under an argon atmosphere, to 4-iodophenylacetic acid (11) (prepared by the process described in Chen, Q.-H. et al., Bioorg. Med. Chem., 14, 7898-7909 (2006)) (1.06 g, 4.05 mmol) was added thionyl chloride (5.00 mL, 68.6 mmol) and heated to reflux (100° C.) for 1.5 h. After cooling to room temperature, the mixture was concentrated under reduced pressure to give 4-iodophenylacetyl chloride (12) as a brown oily crude product.

Under an argon atmosphere, 4-iodophenylacetyl chloride (12) obtained above was dissolved in THF (2 mL) and acetonitrile (2 mL) and cooled to 0° C. To this was slowly added a solution of trimethylsilyldiazomethane in diethyl ether (2.0 M, 4.00 mL, 8.00 mmol), which was stirred overnight (14 h) after warming up to room temperature. After concentrating under reduced pressure, the residue was purified by silica gel flash column chromatography (n-hexane/diethyl ether=1/1) to give 1-diazo-3-(4-iodophenyl)propan-2-one (13) as a pale yellow solid (635 mg, 4.44 mmol, 55.5%, 2 steps).

Comparative Synthesis Example 1-2

1-Diazo-3-(4-iodophenyl)propan-2-one (13) (1.27 g, 4.43 mmol) was dissolved in acetic acid (20 mL) and cooled to 0° C. To this was added 47% hydrobromic acid (1.55 mL, 13.3 mmol) and stirred for an hour after warming to room temperature. After neutralization by adding saturated aqueous solution of sodium bicarbonate, the product was extracted with ethyl acetate (×3) and the organic layer was washed sequentially with saturated aqueous solution of sodium bicarbonate (×1) and saturated brine (×1), and dried over anhydrous sodium sulfate. After filtration, the mixture was concentrated under reduced pressure and the residue was purified by silica gel flash column chromatography (n-hexane/ethyl acetate=7/1) to give 1-bromo-3-(4-iodophenyl)propan-2-one (16) as a colorless solid (1.42 g, 4.18 mmol, 94.4%).

TLC R_(f)=0.41 (n-hexane/ethyl acetate=7/1);

¹H NMR (400 MHz, CDCl₃) δ 3.90 (s, 2H), 3.91 (s, 2H), 6.96-7.01 (AA′BB′, 2H), 7.66-7.71 (AA′BB′, 2H).

Comparative Synthesis Example 1-3

1-Bromo-3-(4-iodophenyl)propan-2-one (16) (460 mg, 1.36 mmol) was dissolved in acetonitrile (4 mL) and to this was added silver nitrate (596 mg, 3.51 mmol) dissolved in acetonitrile (4 mL), and stirred overnight (13 h) at room temperature. The reaction solution was filtered through Celite and the filtrate was washed sequentially with water (×1) and saturated brine (×1), and then the organic layer was dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated under reduced pressure to give 3-(4-iodophenyl)-2-oxopropyl nitrate (17) as a colorless solid (490 mg, <1.36 mmol, <100%), which was used in the next reaction without further purification.

TLC R_(f)=0.26 (tailing) (n-hexane/ethyl acetate=5/1);

¹H NMR (400 MHz, CDCl₃) δ 3.73 (s, 2H), 4.97 (s, 2H), 6.96-7.00 (AA′BB′, 2H), 7.68-7.73 (AA′BB′, 2H).

Comparative Synthesis Example 1-4

3-(4-Iodophenyl)-2-oxopropyl nitrate (17) (450 mg) was dissolved in dimethylsulfoxide (20 mL) and to this was added sodium acetate trihydrate (211 mg, 1.55 mmol), and stirred for an hour at room temperature. To this was added water and the product was extracted with diethyl ether (×3). The organic layer was washed sequentially with water (×1), saturated aqueous solution of sodium bicarbonate (×1) and saturated brine (×1), and then dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated under reduced pressure to give 3-(4-iodophenyl)-2-oxopropanal (18) as an orange solid (346 mg, <1.26 mmol, <90.1%), which was used in the next reaction without further purification.

TLC R_(f)=0.43 (tailing) (n-hexane/ethyl acetate=5/1);

Comparative Synthesis Example 1-5

Under an argon atmosphere, 3-(4-iodophenyl)-2-oxopropanal (18) (343 mg, 1.25 mmol) was dissolved in 1,4-dioxane (2.0 mL) and water (0.4 mL). To this was added coelenteramine (233 mg, 834 mmol) and, after cooling to 0° C., conc. hydrochloric acid (0.20 mL) was added thereto, and then stirred for 6 h at 100° C. After cooling to room temperature, the mixture was extracted with dichloromethane and a small quantity of methanol (×4). The organic layer was washed sequentially with water (×1) and saturated brine (×1) and then dried over anhydrous sodium sulfate. After filtration, the organic layer was concentrated under reduced pressure and the residue was purified in an argon flow by silica gel flash chromatography (deaerated dichloromethane/deaerated methanol=50/1-10/1). The solid obtained was further reprecipitated (n-hexane/acetone) to give i-coelenterazine (15, i-CTZ) as an ocher powder (52.6 mg, 98.6 μmol, 11.8%).

n-CTZ, i-CTZ, me-CTZ, et-CTZ, cf3-CTZ, meo-CTZ, 3me-CTZ, 3meo-CTZ, αmeh-CTZ and 3iso-CTZ prepared above were used for the production of semi-synthetic aequorins and for the analysis of substrate specificity of each luciferase, and so on, in the following EXAMPLES. In addition, coelenterazine (CTZ) and h-coelenterazine (h-CTZ) were used in the following EXAMPLES. CTZ and h-CTZ used were those manufactured by Chisso Corporation.

Hereinafter, CTZ, or h-CTZ, n-CTZ, me-CTZ, et-CTZ, cf3-CTZ, meo-CTZ, 3me-CTZ, 3meo-CTZ, αmeh-CTZ, 3iso-CTZ or i-CTZ is sometimes referred to as coelenterazine or analogs thereof.

Example 1 Production of Semi-Synthetic Aequorins for Substrate Specificity Analysis and Measurement of Luminescence Activity

To produce the following semi-synthetic aequorins, the recombinant apoaequorin manufactured by Chisso Corporation was used. This recombinant apoaequorin was obtained by expressing and purifying according to the method described in Inouye, S, and Sahara, Y. Protein Express. Purif. (2007) 53: 384-389, using piP-H-HE constructed from the expression vector piP-HEAE, which is described in the same literature by inserting histidine sequence therein.

(1) Production of Semi-Synthetic Aequorins

One microliter of 2-mercaptoethanol and 1.31 μg of recombinant apoacquorin solution (made by Chisso Corp.) were added to and mixed with 1 ml of 30 mM Tris-HCl (pH 7.6) containing 10 mM EDTA. Subsequently, 1 μl of a solution of coelenterazine or its analog in ethanol was added and the mixture was allowed to stand at 4° C. to convert into a semi-synthetic aequorin. In order to confirm the regeneration time and regeneration efficiency from apoaequorin to the semi-synthetic aequorin, the luminescence activity was assayed at the respective points of the regeneration process (at the respective points of 0.5, 1, 1.5, 2, 3 and 18 hours from the start of regeneration). The results are shown in FIG. 1.

As shown in FIG. 1, it is demonstrated that coelenterazine analogs of the present invention (me-CTZ, et-CTZ, cf3-CTZ, meo-CTZ, 3me-CTZ, 3meo-CTZ, αmeh-CTZ and 3iso-CTZ) can be luminescence substrates for semi-synthetic aequorins, though luminescent intensities are different.

2) Method for Measuring the Luminescence Activity

Specifically, the luminescence activity described above was measured as follows. The luminescence reaction was started by adding 100 μl of 50 mM Tris-HCl (pH 7.6) containing 50 mM calcium chloride solution to 2 μl of a solution of the semi-synthetic aequorin at each regeneration process. The luminescence activity was measured for 10 seconds with a luminometer Luminescencer-PSN AB2200 (manufactured by Atto Co., Ltd.). The measured values were represented as the maximum intensity (I_(max)) of luminescence. The emission for 60 seconds was integrated, which was made a luminescence capacity.

Example 2 Method for Measuring the Luminescence Patterns and Half Decay Time of Semi-Synthetic Aequorins

A solution of the regenerated semi-synthetic aequorin was diluted to 10-fold with 20 mM Tris-HCl (pH 7.6) containing 0.1% BSA (manufactured by Sigma), 0.01 mM EDTA and 150 mM NaCl. The dilution was dispensed into a 96-well microplate (Nunc #236108) in 5 μl/well. The luminescence reaction was started by injecting 100 μl/well of 50 mM Tris-HCl (pH 7.6) containing 50 mM calcium chloride solution using a luminescence plate reader Centro LB960 (manufactured by Berthold). The luminescence patterns for 60 seconds were measured to determine the half decay time of luminescence (time period to reach the half of the maximum luminescence intensity). The results are shown in FIG. 2 and summarized in TABLE 1 below.

TABLE 1 Luminescence properties of semi-synthetic aequorins Half Maximum Luminescence Luminescence decay emission Coelenterazine activity capacity time wavelength derivative I_(max) (%) 60 sec (%) sec. λ_(max) (nm) Coelenterazine 100.0 100.0 0.81 465.5 h- 87.9 70.1 0.69 465.5 et- 27.3 44.1 1.21 472.0 n- 3.6 24.0 4.71 467.0 cf3- 6.3 48.6 5.95 473.5 i- 5.0 65.3 11.42 472.0 meo- 22.3 60.1 1.63 469.0 3meo- 64.9 35.6 0.52 466.0 me- 5.8 45.0 5.00 469.5 3me- 35.0 21.2 0.51 466.0 αmeh- 2.2 0.9 0.35 467.5 3iso- 51.6 13.4 0.24 464.0

As shown in FIG. 2 and TABLE 1, the half decay time of luminescence was longer than CTZ especially with the semi-synthetic aequorins using me-CTZ and cf3-CTZ as the luminescence substrates. Accordingly, Ca²⁺ can be measured by light being allowed to be emitted slowly, not by instantaneous emission. This indicates that the semi-synthetic aequorins wherein me-CTZ and cf3-CTZ are used as the luminescence substrates are well suited for applications to a high-precision assay system using as an indicator Ca²⁺ level change in the system, in the same manner as in the semi-synthetic aequorins wherein i-CTZ and n-CTZ conventionally known as aequorins having slow half decay time are used as the luminescence substrates.

Example 3 Method for Measuring the Emission Spectra of Semi-Synthetic Aequorins

In a quartz cell with an optical path length of 10 mm, 1 ml of 50 mM Tris-HCl (pH 7.6) containing 1 mM EDTA and 100 μl (100 μg protein) of a solution of the regenerated semi-synthetic aequorin were charged and they were mixed. Subsequently, 100 μl of 50 mM Tris-HCl (pH 7.6) containing 0.1 ml of 10 mM calcium chloride solution was added to the mixture to trigger the luminescence reaction. The spectra were measured on a spectrofluorimeter (FP-6500, manufactured by JASCO Corporation) with the excitation source turned off. The measurement conditions used were as follows: band width, 20 nm; response, 0.5 second and scan speed, 2000 nm/min at 22 to 25° C. The results are shown in FIG. 3. In FIG. 3, AQ denotes semi-synthetic aequorin, for which coelenterazine is the luminescence substrate.

As shown in FIG. 3, the semi-synthetic aequorins, for which me-CTZ, et-CTZ, cf3-CTZ, meo-CTZ, 3me-CTZ, 3meo-CTZ and 3iso-CTZ are the luminescence substrates exhibit different emission spectra from those of known coelenterazine analogs (h-CTZ and i-CTZ).

Example 4 Preparation of Standard Calcium Solution

By dissolving 1 ml of 1 g/L standard calcium carbonate solution (manufactured by Wako Pure Chemical Industry) in 9 ml of 50 mM Tris-HCl (pH 7.6), 10⁻³ M calcium carbonate solution was prepared.

One milliliter of the resulting 10⁻³ M calcium carbonate solution was taken and added to 9 ml of 50 mM Tris-HCl (pH 7.6) to prepare 10⁴M calcium carbonate solution. Furthermore, 3 ml of the resulting 10⁴M calcium carbonate solution was taken and added to 6 ml of 50 mM Tris-HCl (pH 7.6) to prepare 3×10⁻⁴ M calcium carbonate solution. Next, 1 ml of the resulting 10⁻⁴ M calcium carbonate solution was taken and added to 9 ml of 50 mM Tris-HCl (pH 7.6) to prepare 10⁻⁵ M calcium carbonate solution. Further 3 ml of the resulting 10⁻⁵ M calcium carbonate solution was taken and 6 ml of 50 mM Tris-HCl (pH 7.6) was added to prepare 3×10⁻⁵ M calcium carbonate solution. The dilution series was prepared by the serial process above to prepare standard calcium solution of 10⁻³ M to 10⁻⁸ M.

Example 5 Production of Semi-Synthetic Aequorin for Detecting Calcium Levels

After 5 mg of the recombinant apoaequorin (manufactured by Chisso Corp.) was dissolved in 5 ml of 50 mM Tris-HCl (pH 7.6) containing 10 mM DTT and 30 mM EDTA, 100 μg of a solution of 1.2-fold equivalent of coelenterazine analog in ethanol was added thereto. The mixture was allowed to stand at 4° C. overnight to convert into the semi-synthetic aequorin. The semi-synthetic aequorin obtained was concentrated using Amicon Ultra-4 (manufactured by Millipore, molecular weight cut off, 10,000). Subsequently, the concentrate was washed 3 times with 3 ml of 30 mM Tris-HCl (pH 7.6) containing 0.05 mM EDTA to remove an excess of coelenterazine analog and make the EDTA concentration 0.05 mM.

This semi-synthetic aequorin solution (2.5 mg/ml) was diluted with 20 mM Tris-HCl (pH 7.6) containing 0.1% BSA (manufactured by Sigma), 0.01 mM EDTA and 150 mM NaCl.

Example 6 Preparation of Calcium Standard Curve

The calcium standard solution prepared as described above was dispensed into a 96-well microplate (Nunc #236108) in 50 μl/well, and the semi-synthetic aequorin solution diluted was injected in 10 μl/well. The luminescence intensity was measured for 60 seconds using a luminescence plate reader Centro LB960 (manufactured by Berthold) and expressed in terms of the maximum luminescence intensity (I_(max)). The luminescence intensity was measured for each semi-synthetic aequorin in the same fashion. Based on the maximum luminescence intensity (I_(max)) obtained, the calcium standard curve for each semi-synthetic aequorin was prepared.

The results are shown in FIG. 4.

As shown in FIG. 4, it is observed that the photoproteins of the present invention can be used for the detection, quantification or the like of calcium ions, since the calcium standard curves can be prepared by using the semi-synthetic aequorins, which were prepared from coelenterazine analogs of the present invention (me-CTZ, et-CTZ, cf3-CTZ, meo-CTZ, 3me-CTZ, 3meo-CTZ, αmeh-CTZ and 3iso-CTZ).

Example 7 Method for Analyzing the Substrate Specificity and Measuring the Luminescence Activity of the 19 kDa Protein from Oplophorus luciferase

The 19 kDa protein of Oplophorus luciferase was purified by the method described in Inouye, S, and Sasaki, S. Protein Express, and Purif. (2007) 56: 261-268, which was provided for use.

After 1 μl of the 19 kDa protein (2.3 mg/ml) of Oplophorus luciferase containing 1 mM DTT was dissolved in 100 μl of 30 mM Tris-HCl (pH 7.6) containing 10 mM EDTA, 1 μl of a solution of coelenterazine or its analog in ethanol (1 μg/μl) was mixed with the solution to trigger the luminescence reaction. The luminescence activity was measured for 60 seconds using a luminometer Luminescencer-PSN AB2200 (manufactured by Atto Co., Ltd.). The luminescence activity was measured 3 times and expressed as the maximum intensity (I_(max)) of luminescence. The emission for 10 seconds was integrated, which was made a luminescence capacity.

The emission spectra were assayed by adding 5 μg of coelenterazine or its analog (dissolved in 5 μl of ethanol) to 990 μl of 30 mM Tris-HCl (pH 7.6) containing 10 mM EDTA and then adding 20 μl (46 μg) of Oplophorus luciferase thereby to trigger luminescence. The spectra were measured on a spectrofluorimeter (FP-6500, manufactured by JASCO Corporation) with the excitation source turned off, under band width of 20 nm, response of 0.5 second and scan speed of 2000 nm/min at 22 to 25° C. The results of luminescence activity, luminescence capacity and maximum emission wavelength are shown in TABLE 2 below. The emission spectrum charts are shown in FIG. 5.

TABLE 2 Substrate specificity and maximum emission wavelength of Oplophorus luciferase using coelenterazine derivatives Maximum Luminescence Luminescence emission Coelenterazine activity capacity wavelength analog I_(max) (%) 10 secs. (%) λ_(max) (nm) Coelenterazine 100.0 100.0 457.0 h- 68.4 110.2 456.5 i- 32.3 53.8 450.0 et- 21.1 39.5 454.0 cf3- 49.5 68.2 450.5 me- 46.6 82.7 454.4 3me- 80.0 122.8 455.5 meo- 68.1 108.6 454.5 3meo- 189.1 204.7 455.5 αmeh- 15.6 19.0 457.0 3iso- 78.2 71.8 458.0

As shown in TABLE 2, it is observed that me-CTZ, et-CTZ, cf3-CTZ, meo-CTZ, 3me-CTZ, 3meo-CTZ, αme-CTZ and 3iso-CTZ become relatively good luminescence substrates for Oplophorus luciferase. Especially, 3me-CTZ, 3meo-CTZ and 3iso-CTZ exhibit high luminescence activity and/or luminescence capacity when compared to known h-CTZ.

Example 81 Method for Analyzing the Substrate Specificity and Measuring the Luminescence Activity of Gaussia luciferase

Gaussia luciferase was purified by the method described in JPA 2008-099669 and provided for use.

After 1 μl of Gaussia luciferase (0.16 mg/ml) was dissolved in 100 μl of phosphate buffered saline (manufactured by Sigma Inc.) containing 0.01% Tween 20 (manufactured by Sigma Inc.) and 10 mM EDTA, 1 μl of a solution of coelenterazine or its analog dissolved in ethanol (1 μg/Id) was mixed with the solution to trigger the luminescence reaction. The luminescence activity was measured for 10 seconds using a luminometer Luminescencer-PSN AB2200 (manufactured by Atto Co., Ltd.). The luminescence activity was measured 3 times and expressed as the maximum intensity (I_(max)) of luminescence. The emission spectra were assayed by adding 5 μg of coelenterazine or its analog (dissolved in 5 μl of ethanol) to 1000 μl of phosphate buffered saline containing 10 mM EDTA and 0.01% Tween 20, and then adding 1 μl (0.23 μg) of Gaussia luciferase thereby to trigger luminescence. The spectra were measured using a spectrofluorimeter (FP-6500, manufactured by JASCO Corporation) with the excitation source turned off, under band width of 20 nm, response of 0.5 second and scan speed of 2000 nm/min of 20 nm, response of 0.5 second and scan speed of 2000 nm/min at 22 to 25° C. The results of luminescence activity, luminescence capacity and maximum emission wavelength are shown in TABLE 3 below. The emission spectrum charts are shown in FIG. 6.

TABLE 3 Substrate specificity and maximum emission wavelength of Gaussia luciferase using coelenterazine derivatives Maximum Luminescence Luminescence emission Coelenterazine activity capacity wavelength analog I_(max) (%) 10 secs. (%) λ_(max) (nm) Coelenterazine 100.0 100.0 473.0 h- 5.8 7.2 474.0 i- 0.9 1.3 475.0 et- 0.9 1.1 476.0 cf3- 0.8 0.7 476.5 me- 4.5 5.9 475.0 3me- 2.6 2.1 475.5 meo- 13.6 15.5 475.5 3meo- 3.8 3.1 474.5 αmeh- >0.01 >0.01 473.0 3iso- 14.3 7.4 477.0

Gaussia luciferase exhibits high substrate specificity, and any coelenterazine analog which acts as its luminescence substrate is unknown to date. As shown in TABLE 3, it is demonstrated that me-CTZ and 3iso-CTZ become effective luminescence substrates for Gaussia luciferase.

Example 9 Method for Analyzing the Substrate Specificity of Renilla luciferase and Measuring the Luminescence Activity

Renilla luciferase was purified by the method described in Inouye, S. & Shimomura, O. Biochem. Biophys. Res. Commun. (1997) 233: 349-353, which was provided for use.

After 1 μl of Renilla luciferase (0.45 mg/ml) was dissolved in 100 pd of 30 mM Tris-HCl (pH 7.6) containing 10 mM EDTA, 1 μl of a solution of coelenterazine or its analog in ethanol (1 μg/μl) was mixed with the solution to trigger the luminescence reaction. The luminescence activity was measured 3 times using a luminometer Luminescencer-PSN AB2200 (manufactured by Atto Co., Ltd.) and expressed as the maximum intensity (I_(max)) of luminescence. Emission was started by adding 5 μg of coelenterazine or its analog (dissolved in 5 μl of ethanol) to 1000 μl of 30 mM Tris-HCl (pH 7.6) containing 10 mM EDTA and 5 μl (2.3 μg) of Renilla luciferase. The emission spectra were measured on a spectrofluorimeter (FP-6500, manufactured by JASCO Corporation) with the excitation source turned off under band width of 20 nm, response of 0.5 second and scan speed of 2000 nm/min at 22 to 25° C. The results of luminescence activity, luminescence capacity and maximum emission wavelength are shown in TABLE 4 below. The emission spectrum charts are shown in FIG. 7.

TABLE 4 Substrate specificity and maximum emission wavelength of Renilla luciferase using coelenterazine derivatives Maximum Luminescence Luminescence emission Coelenterazine activity capacity wavelength analog I_(max) (%) 10 secs. (%) λ_(max) (nm) Coelenterazine 100.0 100.0 472.5 h- 78.6 90.3 472.5 i- 0.2 0.3 473.0 et- 0.5 0.3 471.0 cf3- 0.8 0.8 474.5 me- 6.6 7.0 472.0 3me- 12.7 13.1 471.5 meo- 8.0 8.6 472.5 3meo- 26.0 31.6 473.5 αmeh- 0.0 0.0 N.D.* 3iso- 25.7 27.5 472.0 N.D.: not detected

As shown in TABLE 4, it is demonstrated that me-CTZ, meo-CTZ, 3meo-CTZ and 3iso-CTZ can be substrates for Renilla luciferase.

[Sequence Listing Free Text]

[SEQ ID NO: 1] Nucleotide sequence of natural apoaequorin [SEQ ID NO: 2] Amino acid sequence of natural apoaequorin [SEQ ID NO: 3] Nucleotide sequence of natural apoclytin-I [SEQ ID NO: 4] Amino acid sequence of natural apoclytin-I [SEQ ID NO: 5] Nucleotide sequence of natural apoclytin-II [SEQ ID NO: 6] Amino acid sequence of natural apoclytin-II [SEQ ID NO: 7] Nucleotide sequence of natural apomitrocomin [SEQ ID NO: 8] Amino acid sequence of natural apomitrocomin [SEQ ID NO: 9] Nucleotide sequence of natural apobelin [SEQ ID NO: 10] Amino acid sequence of natural apobelin [SEQ ID NO: 11] Nucleotide sequence of natural apobervoin [SEQ ID NO: 12] Amino acid sequence of natural apobervoin [SEQ ID NO: 13] Nucleotide sequence of Renilla luciferase [SEQ ID NO: 14] Amino acid sequence of Renilla luciferase [SEQ ID NO: 15] Nucleotide sequence of Oplophorus luciferase [SEQ ID NO: 16] Amino acid sequence of Oplophorus luciferase [SEQ ID NO: 17] Nucleotide sequence of Gaussia luciferase [SEQ ID NO: 18] Amino acid sequence of Gaussia luciferase 

1: A compound represented by general formula (1) below: wherein:

R¹ is hydrogen, hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl; R² is hydrogen, hydroxy, a halogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl; R³ is hydrogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxyl; R⁴ is a substituted or unsubstituted aryl, a substituted or unsubstituted arylalkyl, a substituted or unsubstituted arylalkenyl, an alkyl which may optionally be substituted with an alicyclic group, an alkenyl which may optionally be substituted with an alicyclic group, an alicyclic group, or a heterocyclic group; R⁵ is hydrogen or a substituted or unsubstituted alkyl; X¹ is hydrogen, hydroxy, a halogen, an alkoxyl or amino; and, X² is hydrogen or hydroxy; with the proviso that when R² and R³ are hydrogen, R¹ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl, when R¹ and R³ are hydrogen, R² is hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl, and, when R¹ and R² are hydrogen, R³ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxyl. 2-14. (canceled)
 15. A process for producing a compound represented by general formula (1) below:

wherein R¹, R², R³, R⁴, R⁵, X¹ and X² are the same as defined below, which comprises reacting a compound represented by general formula (2) below:

wherein: R⁴ is a substituted or unsubstituted aryl, a substituted or unsubstituted arylalkyl, a substituted or unsubstituted arylalkenyl, an alkyl which may optionally be substituted with an alicyclic group, an alkenyl which may optionally be substituted with an alicyclic group, an alicyclic group, or a heterocyclic group: R⁵ is hydrogen or a substituted or unsubstituted alkyl; X¹ is hydrogen, hydroxy, a halogen, an alkoxyl or amino; and, X² is hydrogen or hydroxy; with a compound represented by general formula (3) below:

wherein: R¹ is hydrogen, hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl; R² is hydrogen, hydroxy, a halogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl or an alkoxyl; and, R³ is hydrogen, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxy; with the proviso that when R² and R³ are hydrogen, R¹ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl, when R¹ and R³ are hydrogen, R² is hydroxy, an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, trifluoromethyl, or an alkoxyl, and, when R¹ and R² are hydrogen, R³ is an alkyl having 1 to 4 carbon atoms which may optionally be substituted with an alicyclic group, or an alkoxyl.
 16. The process according to claim 15, wherein R¹ is hydrogen, hydroxy, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, trifluoromethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, in the general formula (1).
 17. The process according to claim 15, wherein R² is hydrogen, hydroxy, fluorine, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, trifluoromethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, in the general formula (1).
 18. The process according to claim 15, wherein R³ is hydrogen, methyl, ethyl, propyl, adamantylmethyl, cyclopentylmethyl, cyclohexylmethyl, cyclohexylethyl, methoxy, ethoxy, n-propoxy, iso-propoxy, sec-propoxy, n-butoxy, iso-butoxy, sec-butoxy or tert-butoxy, in the general formula (1).
 19. The process according to claim 15, wherein R⁴ is phenyl, p-hydroxyphenyl, benzyl, α-hydroxybenzyl, phenylethyl, phenylvinyl, cyclohexyl, cyclohexylmethyl, cyclohexylethyl, methyl, ethyl, propyl, 2-methylpropyl, 2-methylpropenyl, adamantylmethyl, cyclopentylmethyl or thiophen-2-yl, in the general formula (1).
 20. The process according to claim 15, wherein, in the general formula (1): R¹ is hydrogen, methyl, ethyl, trifluoromethyl or methoxy, R² is hydrogen, hydroxy, methyl or methoxy, R³ is hydrogen or methyl, R⁴ is benzyl, R⁵ is hydrogen, X¹ is hydroxy, and, X² is hydrogen, with the proviso that when R² and R³ are hydrogen, R¹ is methyl, ethyl, trifluoromethyl or methoxy, when R¹ and R³ are hydrogen, R² is hydroxy, methyl or methoxy, and, when R¹ and R² are hydrogen, R³ is methyl.
 21. A method for producing a calcium-binding photoprotein, which comprises contacting the compound according to claim 1 with an apoprotein of the calcium-binding photoprotein to obtain the calcium-binding photoprotein.
 22. A method for detecting or quantifying a calcium ion, which comprises using the calcium-binding photoprotein produced by the method according to claim
 21. 23. A method for analyzing a physiological function or enzyme activity, which comprises performing a bioluminescence resonance energy transfer (BRET) assay using as a donor protein the calcium-binding photoprotein produced by the method according to claim
 21. 24. A method for measuring a transcription activity or detecting an analyte, which comprises using the compound according to claim 1, and a luciferase derived from Renilla sp., Oplophorus sp. or Gaussia sp.
 25. A method for analyzing a physiological function or enzyme activity, which comprises performing a bioluminescence resonance energy transfer (BRET) assay using the compound according to claim 1 and a luciferase derived from Renilla sp., Oplophorus sp. or Gaussia sp. as a donor protein.
 26. The method according to claim 24, wherein Renilla sp. is Renilla reniformis.
 27. The method according to claim 26, wherein the luciferase derived from Renilla reniformis comprises a polypeptide consisting of the amino acid sequence of SEQ ID NO:
 14. 28. The method according to claim 24, wherein Oplophorus sp. is Oplophorus grachlorostris.
 29. The method according to claim 28, wherein the luciferase derived from Oplophorus grachlorostris comprises a polypeptide consisting of the amino acid sequence of SEQ ID NO:
 16. 30. The method according to claim 24, wherein Gaussia sp. is Gaussia princeps.
 31. The method according to claim 30, wherein the luciferase derived from Gaussia princeps comprises a polypeptide consisting of the amino acid sequence of SEQ ID NO:
 18. 32. A kit for measuring a transcription activity or detecting an analyte, comprising the compound according to claim 1 and a luciferase derived from at least one organism selected from the group consisting of Renilla sp., Oplophorus sp. and Gaussia sp.
 33. A kit for analyzing a physiological function or enzyme activity, comprising the compound according to claim 1, a luciferase derived from at least one organism selected from the group consisting of Renilla sp., Oplophorus sp. and Gaussia sp. and at least one selected from the group consisting of an organic compound and a fluorescent protein, utilizing the principle of intermolecular interaction by a bioluminescence resonance energy transfer (BRET) assay. 